So, I'm a big fan of Vanguards and I love to design cards, so I imagined what many of the Guild Leaders and Champions could look as Vanguard cards. These cards are designed for casual edh play in mind, and they are currently tested in my playgroup which like the idea and mechanics behind.
So, here's my take. Guild leaders all have a common pattern in their design with abilities that make feel in flavor and mechanic as they belong to that characters as much as possible (H is Hand size and L is starting life points):
Niv-Mizzet RU : - deals 1 damage to any target. Draw a card. (U/R)(U/R)(U/R)(U/R) : Each player discards their hand and draws seven cards.
H:+3 L:-3
Trostani WG: Create a green Saproling token 1/1. Gain 1 life. (G/W)(G/W)(G/W) : For each token with different names you control, populate.
H:0 L:+4
Savra BG: You may sacrifice a nontoken creature. If you do, target opponent sacrifice a creature, then gain 1 life. (B/G)(B/G)(B/G)(B/G) : You may discard a creature card. If you do, search your library for a creature card, reveal it, and put it into your hand. Then shuffle your library.
H:-1 L:-1
Zegana UG: Put a +1/+1 counter up to one target creature. Draw a card. (G/U)(G/U)(G/U)(G/U) : Proliferate
H:+3 L:-5
Aurelia RW: - deals 1 damage to any target. Gain 1 life. (R/W)(R/W)(R/W)(R/W)(R/W): Untap all creatures that attacked this turn. After this main phase, there is an additional combat phase.
H:0 L:+2
Augustin IV: WU: Counter target spell unless its controller pays 1. Gain 1 life. (W/U)(W/U)(W/U)(W/U): Detain target nonland permanent.
H:+2 L:+0
Judith RB: Target opponent lose 1 life. - deals 1 damage to any target. (B/R)(B/R)(B/R): Creatures you control gain +1/+0 and menace until the end of turn.
H:-1 L:-5
Lazav UB: Target opponent discard a card and you draw a card. Activate this ability only in your turn. (U/B)(U/B)(U/B)(U/B)(U/B): Create a creature token that is a copy of target creature, except his name is Lavaz and is legendary.
H:+2 L:-5
Kaya WB: Exile target card from an opponent's graveyard, then that player lose 1 life. (W/B)(W/B)(W/B)(W/B)(W/B): Each opponent exile a creature he or she controls.
H:+1 L:+2
Rakdos RB: Each opponent discards a card. - deals 1 damage to any target. Activate this ability only on your turn. (B/R)(B/R)(B/R)(B/R)(B/R) : Each player loses half their lifes, rounded up.
H:-1 L:-7
Svogthir BG: Put the top three cards of your library into your graveyard. Return up to one target land card from your graveyard to the battlefield tapped. (B/G)(B/G)(B/G): Return target creature card from your graveyard to your hand.
H:-1 L:-2
Azor UW: Draw a card and gain 1 life. (W/U)(W/U)(W/U): Each opponent can’t cast instant or sorcery spells this turn.
H:+3 L:+3
Szadek UB: Each opponent puts the top card of their library into their graveyard and lose 1 life. (U/B)(U/B)(U/B)(U/B)(U/B): You may discard a card. If you do, search in your library for a card with the same converted mana cost, reveal it and put it in your hand. Then shuffle your library.
H:+2 L:-7
Razia RW: Prevent the next 1 damage that would be dealt to any target this turn. - Deals 1 damage to any target. (R/W)(R/W)(R/W): Creatures you control gain +1/+0 and first strike until the end of turn.
H:0 L:+4
Mat'Selesnya WG: Create a green Saproling token 1/1 then put a +1/+1 counter to one target creature. (G/W)(G/W)(G/W): Creatures you control gain +0/+1 and hexproof until the end of turn.
H:0 L:+8
Domri Rade GR: You may sacrifice an artifact or enchantment. If you do, destroy target artifact or enchantment. (R/G)(R/G)(R/G): Target creature you control fight another target creature.
H:-1 L:-5
Ral Zarek UR: Draw a card, then add a mana of any color. You may spend this mana only to cast instant or sorceries. (U/R)(U/R)(U/R)(U/R): You may cast target instant or sorcery from your hand without paying it's mana cost.
Momir Vig UG: Draw a card, then add a mana of any color. You may spend this mana only to cast creature spells or activate abilities of creatures. (G/U)(G/U)(G/U)(G/U) : Draw cards equal to the greatest power among creatures you control.
H:+3 L:-5
Obzedat WB: Each opponent loses 1 life and you gain that much life. (W/B)(W/B)(W/B)(W/B)(W/B): Return target creature card from a graveyard to the battlefield. If it would leave the battlefield, exile it instead of putting it anywhere else.
H:+1 L:+8
Borborygmos RG: - deals 1 damage to each player, each creature and each planeswalker. (R/G)(R/G)(R/G)(R/G)(R/G): Destroy target land or artifact.
H:-1 L:+6
Tibor and Lumia RR: Choose new targets or modes for target permanent or spell. Activate this ability only once each turn. UU: Copy target instant or sorcery spell. You may choose new targets for the copy. Activate this ability only once each turn.
H:+2 L:-2
Emmara Tandris WW: Prevent the next 2 damages that would be dealt to target creature this turn. Activate this ability only once for turn. GG: Reveal a creature card from your hand. Has convoke until the end of turn. Activate this ability only once for turn.
H:0 L:+5
Jarad BB: Search your library for a creature card and put that card into your graveyard. Then shuffle your library. Activate this ability only once for turn. GG: Pur target creature card from your graveyard on top of your library. Activate this ability only once for turn.
H:-1 L:-4
Exava RR:Target creature gain haste until the end of turn. Activate this ability only once for turn. BB: Target creature gain first strike until the end of turn. Activate this ability only once for turn.
H:0 L:-5
Teysa Karlov BB: Put a -1/-1 counter on target creature. Activate this ability only once for turn. WW: Create a white spirit token 1/1 with flying. Activate this ability only once for turn.
H:+1 L:-1
Lexya, Lydia and Ludmilla GG: Target creature must be blocked this turn. Activate this ability only once for turn. BB: Target creature gain death touch until the end of turn. Activate this ability only once for turn.
H:0 L:0
Tajic WW: Target creatures gain indestructible until the end of turn. Activate this ability only once for turn. RR: Target creature gain double strike until the end of turn. Activate this ability only once for turn.
H:0 L:+4
Melek UU: Target instant card in your graveyard has flashback until the end of turn. Activate this ability only once for turn. RR: Target sorcery card in your graveyard has flashback until the end of turn. Activate this ability only once for turn.
H:+1 L:-3
Tolsimir WW: Creatures you control gain +1/+1 until the end of turn. Activate this ability only once for turn. GG: Create a green Wolf token creature 2/2. Activate this ability only once for turn.
H:0 L:+5
Mirko Vosk UU: Target opponent puts the top three cards of their library into their graveyard. Activate this ability only once for turn. BB: Target opponent lose life and you gain life equal to the cards put into that opponent graveyard from anywhere this turn. Activate this ability only once for turn.
H:+1 L:-2
Etrata UU: Target creature can't be blocked this turn. Activate this ability only once for turn. BB: Destroy target creature to the defending player which was dealt damage from a creature you control this turn. Activate this ability only once for turn.
H:+1 L:-4
Lavinia WW: Target creature cannot attack or block this turn. Activate this ability only once for turn. UU: Counter target activated or triggered ability. Activate this ability only once for turn.
H:+1 L:+1
Isperia WW: Target creature gains vigilance until the end of turn. Activate this ability only once for turn. UU: Target creatures gains flying until the end of turn. Activate this ability only once for turn.
H:+3 L:+3
Vannifar GG: You may sacrifice a creature. If you do, search your library for a creature card with converted mana cost equal to 1 plus the sacrificed creature’s converted mana cost, put that card in your hand, then shuffle your library. Activate this ability only once for turn. UU: You may sacrifice a creature. If you do, draw cards equal to the converted mana cost of that creature, plus one. Activate this ability only once for turn.
H:+2 L:-4
Vorel GG: Put a +1/+1 counter on target creature. Activate this ability only once for turn. UU: Add a counter of a type already present on target permanent or player. Activate this ability once for turn.
H:0 L:-2
Lyzolda RR: You may sacrifice a creature. If you do, - deals damage equal to the sacrificed creature’s power to any target. Activate this ability only once for turn. BB: You may sacrifice a creature. If you do, draw X cards and lose X life equal the sacrificed creature's converted mana cost. Activate this ability only once for turn.
H:0 L:-5
Nikya GG: Whenever you tap a land for mana this turn, add one mana of any type that land produced. Activate this ability only once for turn. RR: You may pay X. If yoy do, - deals X damages to target creature or planeswalker.
H:+2 L:-5
Karlov WW: Gain 2 lifes. Activate this ability only once for turn. BB: Target opponent lose 2 life. Activate this ability only once for turn.
H:+1 L:+6
Ruric Thar GG: Target creature gain hexproof until the end of turn. Activate this ability only once for turn. RR: Targaet creature spell can't be countered this turn. Activate this ability only once for turn.
H:-1 L:+3
Feather RR: Creatures with flying you control gain haste and double strike until the end of turn. WW: Creatures with haste you control gains lifelink and vigilance until the end of turn
H:0 L:-3
So, here's my take. Guild leaders all have a common pattern in their design with abilities that make feel in flavor and mechanic as they belong to that characters as much as possible (H is Hand size and L is starting life points):
Niv-Mizzet
RU : - deals 1 damage to any target. Draw a card.
(U/R)(U/R)(U/R)(U/R) : Each player discards their hand and draws seven cards.
H:+3 L:-3
Trostani
WG: Create a green Saproling token 1/1. Gain 1 life.
(G/W)(G/W)(G/W) : For each token with different names you control, populate.
H:0 L:+4
Savra
BG: You may sacrifice a nontoken creature. If you do, target opponent sacrifice a creature, then gain 1 life.
(B/G)(B/G)(B/G)(B/G) : You may discard a creature card. If you do, search your library for a creature card, reveal it, and put it into your hand. Then shuffle your library.
H:-1 L:-1
Zegana
UG: Put a +1/+1 counter up to one target creature. Draw a card.
(G/U)(G/U)(G/U)(G/U) : Proliferate
H:+3 L:-5
Aurelia
RW: - deals 1 damage to any target. Gain 1 life.
(R/W)(R/W)(R/W)(R/W)(R/W): Untap all creatures that attacked this turn. After this main phase, there is an additional combat phase.
H:0 L:+2
Augustin IV:
WU: Counter target spell unless its controller pays 1. Gain 1 life.
(W/U)(W/U)(W/U)(W/U): Detain target nonland permanent.
H:+2 L:+0
Judith
RB: Target opponent lose 1 life. - deals 1 damage to any target.
(B/R)(B/R)(B/R): Creatures you control gain +1/+0 and menace until the end of turn.
H:-1 L:-5
Lazav
UB: Target opponent discard a card and you draw a card. Activate this ability only in your turn.
(U/B)(U/B)(U/B)(U/B)(U/B): Create a creature token that is a copy of target creature, except his name is Lavaz and is legendary.
H:+2 L:-5
Kaya
WB: Exile target card from an opponent's graveyard, then that player lose 1 life.
(W/B)(W/B)(W/B)(W/B)(W/B): Each opponent exile a creature he or she controls.
H:+1 L:+2
Rakdos
RB: Each opponent discards a card. - deals 1 damage to any target. Activate this ability only on your turn.
(B/R)(B/R)(B/R)(B/R)(B/R) : Each player loses half their lifes, rounded up.
H:-1 L:-7
Svogthir
BG: Put the top three cards of your library into your graveyard. Return up to one target land card from your graveyard to the battlefield tapped.
(B/G)(B/G)(B/G): Return target creature card from your graveyard to your hand.
H:-1 L:-2
Azor
UW: Draw a card and gain 1 life.
(W/U)(W/U)(W/U): Each opponent can’t cast instant or sorcery spells this turn.
H:+3 L:+3
Szadek
UB: Each opponent puts the top card of their library into their graveyard and lose 1 life.
(U/B)(U/B)(U/B)(U/B)(U/B): You may discard a card. If you do, search in your library for a card with the same converted mana cost, reveal it and put it in your hand. Then shuffle your library.
H:+2 L:-7
Razia
RW: Prevent the next 1 damage that would be dealt to any target this turn. - Deals 1 damage to any target.
(R/W)(R/W)(R/W): Creatures you control gain +1/+0 and first strike until the end of turn.
H:0 L:+4
Mat'Selesnya
WG: Create a green Saproling token 1/1 then put a +1/+1 counter to one target creature.
(G/W)(G/W)(G/W): Creatures you control gain +0/+1 and hexproof until the end of turn.
H:0 L:+8
Domri Rade
GR: You may sacrifice an artifact or enchantment. If you do, destroy target artifact or enchantment.
(R/G)(R/G)(R/G): Target creature you control fight another target creature.
H:-1 L:-5
Ral Zarek
UR: Draw a card, then add a mana of any color. You may spend this mana only to cast instant or sorceries.
(U/R)(U/R)(U/R)(U/R): You may cast target instant or sorcery from your hand without paying it's mana cost.
Momir Vig
UG: Draw a card, then add a mana of any color. You may spend this mana only to cast creature spells or activate abilities of creatures.
(G/U)(G/U)(G/U)(G/U) : Draw cards equal to the greatest power among creatures you control.
H:+3 L:-5
Obzedat
WB: Each opponent loses 1 life and you gain that much life.
(W/B)(W/B)(W/B)(W/B)(W/B): Return target creature card from a graveyard to the battlefield. If it would leave the battlefield, exile it instead of putting it anywhere else.
H:+1 L:+8
Borborygmos
RG: - deals 1 damage to each player, each creature and each planeswalker.
(R/G)(R/G)(R/G)(R/G)(R/G): Destroy target land or artifact.
H:-1 L:+6
Tibor and Lumia
RR: Choose new targets or modes for target permanent or spell. Activate this ability only once each turn.
UU: Copy target instant or sorcery spell. You may choose new targets for the copy. Activate this ability only once each turn.
H:+2 L:-2
Emmara Tandris
WW: Prevent the next 2 damages that would be dealt to target creature this turn. Activate this ability only once for turn.
GG: Reveal a creature card from your hand. Has convoke until the end of turn. Activate this ability only once for turn.
H:0 L:+5
Jarad
BB: Search your library for a creature card and put that card into your graveyard. Then shuffle your library. Activate this ability only once for turn.
GG: Pur target creature card from your graveyard on top of your library. Activate this ability only once for turn.
H:-1 L:-4
Exava
RR:Target creature gain haste until the end of turn. Activate this ability only once for turn.
BB: Target creature gain first strike until the end of turn. Activate this ability only once for turn.
H:0 L:-5
Teysa Karlov
BB: Put a -1/-1 counter on target creature. Activate this ability only once for turn.
WW: Create a white spirit token 1/1 with flying. Activate this ability only once for turn.
H:+1 L:-1
Lexya, Lydia and Ludmilla
GG: Target creature must be blocked this turn. Activate this ability only once for turn.
BB: Target creature gain death touch until the end of turn. Activate this ability only once for turn.
H:0 L:0
Tajic
WW: Target creatures gain indestructible until the end of turn. Activate this ability only once for turn.
RR: Target creature gain double strike until the end of turn. Activate this ability only once for turn.
H:0 L:+4
Melek
UU: Target instant card in your graveyard has flashback until the end of turn. Activate this ability only once for turn.
RR: Target sorcery card in your graveyard has flashback until the end of turn. Activate this ability only once for turn.
H:+1 L:-3
Tolsimir
WW: Creatures you control gain +1/+1 until the end of turn. Activate this ability only once for turn.
GG: Create a green Wolf token creature 2/2. Activate this ability only once for turn.
H:0 L:+5
Mirko Vosk
UU: Target opponent puts the top three cards of their library into their graveyard. Activate this ability only once for turn.
BB: Target opponent lose life and you gain life equal to the cards put into that opponent graveyard from anywhere this turn. Activate this ability only once for turn.
H:+1 L:-2
Etrata
UU: Target creature can't be blocked this turn. Activate this ability only once for turn.
BB: Destroy target creature to the defending player which was dealt damage from a creature you control this turn. Activate this ability only once for turn.
H:+1 L:-4
Lavinia
WW: Target creature cannot attack or block this turn. Activate this ability only once for turn.
UU: Counter target activated or triggered ability. Activate this ability only once for turn.
H:+1 L:+1
Isperia
WW: Target creature gains vigilance until the end of turn. Activate this ability only once for turn.
UU: Target creatures gains flying until the end of turn. Activate this ability only once for turn.
H:+3 L:+3
Vannifar
GG: You may sacrifice a creature. If you do, search your library for a creature card with converted mana cost equal to 1 plus the sacrificed creature’s converted mana cost, put that card in your hand, then shuffle your library. Activate this ability only once for turn.
UU: You may sacrifice a creature. If you do, draw cards equal to the converted mana cost of that creature, plus one. Activate this ability only once for turn.
H:+2 L:-4
Vorel
GG: Put a +1/+1 counter on target creature. Activate this ability only once for turn.
UU: Add a counter of a type already present on target permanent or player. Activate this ability once for turn.
H:0 L:-2
Lyzolda
RR: You may sacrifice a creature. If you do, - deals damage equal to the sacrificed creature’s power to any target. Activate this ability only once for turn.
BB: You may sacrifice a creature. If you do, draw X cards and lose X life equal the sacrificed creature's converted mana cost. Activate this ability only once for turn.
H:0 L:-5
Nikya
GG: Whenever you tap a land for mana this turn, add one mana of any type that land produced. Activate this ability only once for turn.
RR: You may pay X. If yoy do, - deals X damages to target creature or planeswalker.
H:+2 L:-5
Karlov
WW: Gain 2 lifes. Activate this ability only once for turn.
BB: Target opponent lose 2 life. Activate this ability only once for turn.
H:+1 L:+6
Ruric Thar
GG: Target creature gain hexproof until the end of turn. Activate this ability only once for turn.
RR: Targaet creature spell can't be countered this turn. Activate this ability only once for turn.
H:-1 L:+3
Feather
RR: Creatures with flying you control gain haste and double strike until the end of turn.
WW: Creatures with haste you control gains lifelink and vigilance until the end of turn
H:0 L:-3