MTG Salvation

Here. Have a nightmare for record-keeping.

Sorchar, the Kindler WUBRG
Legendary Creature - Elemental Shaman
Sacrifice a creature: Add one mana of any of the sacrificed creature's colors to your mana pool.
2: Put a 1/1 white Elemental creature token with vigilance, a 1/1 blue Elemental creature token with flying, a 1/1 red Elemental creature token with lifelink, a 1/1 red Elemental creature token with haste, or a 1/1 green Elemental creature token with hexproof onto the battlefield.
(5/5)

Next: Create a commander with all symmetrical abilities.

See the templating for Notion Thief for the last ability (although it's pretty wordy). I'm not sure if the ability leans more toward to blue or black side of the spectrum, but I suppose it's alright here.

Crown of Envy 3
Artifact - Equipment
As long as an opponent has more life than you, equipped creature gets +5/+0 and has haste.
Equip - Pay 2 life.

I'm not sure where to put any of the numbers on this one. Kind of a dethrone-ish equipment.

Nearly unstoppable, but also pretty impossible to voltron:

Baruun, Relentless Hound 2RGW
Legendary Creature - Hound
Hexproof, haste
~ must be blocked if able.
If ~ would leave the battlefield, exile it. Return ~ to the battlefield at the beginning of your next combat phase tapped and attacking.
"One by one, we will track you down."
(7/2)

Next: Make an upgraded version of an existing creature that you wish could be used as a commander.

I really like Zedruu-esque political trickery. Multiplayer shenanigans ensue:

Aryllen, Travesty of Law 1URW
Planeswalker - Aryllen
+1: Put two 0/1 white Goat creature tokens onto the battlefield.
-2: Exchange control of two target permanents that share a card type.
-5: You get an emblem with "Permanents you control that another player owns have hexproof and indestructible."
(3)

Mage of the Steppe + Contamination

Magus of the Eclipse 2WB
Creature - Human Wizard
At the beginning of your upkeep, exile a nontoken permanent you control, then search your library for a Swamp card and put it onto the battlefield tapped.
(2/2)

Maybe it makes you laugh. Maybe it doesn't. Sheep.

Steelwool Shepard GWU
Legendary Creature - Human Cleric
Other creatures you control are 0/1 white Sheep creatures with no abilities.
Sheep and Goat creatures you control are indestructible.
(1/5)

Next: Any "evil" legendary of your choice turned to their opposite colors.

If you had a returning mechanic for this hypothetical block, you might consider landfall. I could see a deck built around getting a benefit from playing lands, and then exiling them for another landfall later.

Vallo the Timekeeper 2UU
Planeswalker - Vallo
+2: Remove a counter from target permanent or suspended card.
0: Exile a spell from your hand with X time counters on it, where X is that spell's converted mana cost.
-8: Remove all counters from any number of target permanents and/or suspended cards.
(2)

Next: The result of an Ooze getting big enough and smart enough that it achieves a spark and sense of identity.

ThePeople'sHero is pretty spot on for where I see this ability being balanced at. The only minor change that I would make to their suggestion is to add a little bit more choice to the ability, in the same way that something like Devour gives you the option of sacrificing less or more. A simple "up to" is all that is required here:

Territorial # (At the beginning of your upkeep, you may exile up to # lands you control. This creature gets +1/+1 for each land card exiled with it. If this creature would leave the battlefield or you would lose control of it, return the exiled cards to the battlefield under their owner's control tapped.)

Optionally, you might consider clarifying the end of the reminder to avoid any possible confusion with rules:

Territorial # (At the beginning of your upkeep, you may exile up to # lands you control. This creature gets +1/+1 for each land card exiled with it. If this creature would leave the battlefield or you would lose control of it, return all lands exiled by this creature to the battlefield under their owner's control tapped.)

It's a fairly wordy ability, but the reminder text should only be required for commons and uncommons, leaving plenty of room for other abilities for fancy legendaries. Since the ability counts all cards exiled by the creature, not just ones exiled by the ability specifically, I'd like to see some powerful creatures that use their territory in other ways as well. Like:

Extra Territory Creature (cost)
Creature - Something
Territorial 2
T, exile a land you control: ~ deals 2 damage to target creature or player.
(P/T)

Territory Mana Creature (cost)
Creature - Something
Territorial 1
Whenever you tap a land with the same name as a land exile by ~ for mana, add one mana of any color that land produced to your mana pool.
(P/T)

Just some additional ways to benefit from the territorial ability.

Don't keep up on specific players, so I gotta go with the first option.

Riftsea Guardian 4UU
Legendary Creature - Giant Oyster
You may choose not to untap ~ during your untap step.
Whenever three or more creatures attack you or a planewalker you control, you may untap ~.
W, T: Tap all attacking creatures. Those creatures don't untap during their controller's untap step for as long as ~ remains tapped.
(2/6)

Next: A commander that goes well with ETB effects in a way other than flickering.

This doesn't fit Azorious at all. You can justify it to a small extend with flavor, but mechanically it doesn't really do what you expect. With a name like "diplomacy," you would expect interesting choices and political strategy. Any creature with Diplomacy will just always attack, which doesn't feel very Azorious.

Skyseed Harbinger 1GGWW
Creature - Saproling Angel
Saprolings you control have flying.
Whenever ~ or another Saproling enters the battlefield under your control, you gain 1 life.
(4/4)

Next: Plant Demon

Ukgreth the Planeburner 1BRG
Legendary Creature - Giant Shaman
Discard a card: Until end of turn, whenever target land is tapped for mana it adds an additional 1 to its controller's mana pool and deals 2 damage to its controller.
Whenever a land is put into your graveyard from anywhere, add one mana of any color to your mana pool. Use this mana only to cast spells or activated abilities from graveyards.
(4/4)

Next: A commander that needs multiple drawbacks to be balanced.

The Slavehorde is incredibly broken and could never see print. No one would ever pay for X. Instead, they just put out a 1-mana 3/2 pro-everything with annihilator. That basically wins the game on turn 1.

Anyways:

Sliver Insectivore GG
Creature - Sliver Anteater
Slivers you control have "Whenever this creature blocks or becomes blocked by an Insect or Spider, destroy that creature and put a +1/+1 counter on this creature."
(2/2)

Next: Spider Dragon

Dirious, Sparkdancer RW
Legendary Creature - Human Wizard
Vigilance
When ~ enters the battlefield, choose another target creature you control.
As long as ~ is on the battlefield, the chosen creature has "T, tap another untapped creature you control: This creature deals 3 damage to target player."
R, : ~ deals 3 damage to target player.
(2/2)

Next: A commander uses other players' mana pools in a fair way.