Choose two guilds that share a color, then design a common mono-colored card of that shared color which shows synergy between at least one of each guild's mechanics.
Necrogenic Scion (Common) G
Creature - Elf Druid
1/1
If an effect would place one or more +1/+1 counters on Necrogenic Scion, it places twice as many on it instead.
A one-creature Corpsejack Menance. Solid, but not really innovative. Both guilds use counters so this does fit that requirement, but I don't think you'd see it at common.
Azorio-Orzhov
Trusted Advokist1W
Creature - Human Wizard
Whenever a permanent becomes the target of a spell or ability, add W to your mana pool.
2/2
So the white mana is designed to pay for extort costs, and I guess technically detain does target. Problems are A) the mana can be used for anything and mana production isn't white, B) open ended target triggers like this are relatively easy to abuse, e.g. this plus Shuko gives infinite mana on turn two.
Dimir and Rakdos. I always knew those crazy kids would get together
Callous Slayer 3B
Creature - Human Assassin {C}
Callous Slayer can’t block and is
unblockable.
Callous Slayer gets -1/-1 for each
card in your hand. Unthinking. Unfeeling. Unconcerned
with the sanctity of life.
7/7
Unblockable to serve as a Cipher target and it wants (ideally) zero cards in hand to play with Hellbent.
Putting "{C}" on a card doesn't make in common. This is just too large and complex for a black common creature, and if I'm getting in for 4-7 unblockable damage, I'm not going to care much about adding cipher spells to it as well.
Pair of Thugs2RR [C]
Sorcery
Put two 1/1 red Human Berserker creature tokens with unleash onto the battlefield. "You can buy anything on Tin Street Alley. Weapons, drugs, even men. When you're here, everything has a price and a catch."
—Arrester Lavinia, Tenth Precinct
Unleash for Rakdos, tokens for Batallion.
Counters on tokens are a necessary evil, but imo ought not be forced to happen like this. I do like the design, how it works with the guilds, and how you refrained from giving the tokens haste as well,
Let's go with Selesnya + Boros.
Recruitment Officer2W
Creature - Human Soldier {C} Battalion - Whenever Recruitment Officer and at least two other creatures attack, put a 1/1 white Soldier creature token onto the battlefield tapped and attacking. 2/2 Even the Selesnya have their need for cops. They're just more subtle in their methods of selection.
Solid card and an ability I'm surprised didn't see print, though I'd expect to see it at uncommon. Doesn't really work with Selesnya, though, as you really want to be populate something more than a 1/1 non-flyer.
Gruul/Simic
Inspirational Infusion1G
Instant (C)
Target creature gets +X/+X until end of turn where X is the total amount of +1/+1 counters on permanents you control. When you are truly part of a group, you can feel their strength pushing you into greatness.
Nice card, fits both guilds pretty well and I can see it at common. Possibly should be 2G, and the link to Gruul could be a bit stronger.
+
Cephalovortex2U
Creature — Octopus Weird
Graft 1
Whenever an instant or sorcery spell you control resolves, put a +1/+1 counter on Cephalovortex.
0/0
Interesting card. I think I like it. Might have too much going on for common, but I can see it just getting under that bar. You definitely found a nice sweet spot between Izzet and Simic.
I'm surprised no one is going Dimir-Izzet
Chemister's Informant2U Creature - Human Rogue (C)
Whenever you cast an instant or sorcery spell, Chemister's Informant is unblockable this turn.
2/2
Cool card. Feels a lot more Dimir than Izzet. With this guy around, a cipher spell is almost guaranteed to hit twice. Still, well costed and simple enough for common.
Choose two guilds that share a color, then design a common mono-colored card of that shared color which shows synergy between at least one of each guild's mechanics.
Golgari BG //gatherer.wizards.com/Pages/Search/Default.aspx?action=advanced&text=+[Scavenge">&set=[%22Return%20to%20Ravnica%22]&color=+[G]"]Scavenge
Simic UG //gatherer.wizards.com/Pages/Search/Default.aspx?text=+[graft">&set=[%22Dissension%22]"]Graft]
Necrogenic Scion (Common)
G
Creature - Elf Druid
1/1
If an effect would place one or more +1/+1 counters on Necrogenic Scion, it places twice as many on it instead.
A one-creature Corpsejack Menance. Solid, but not really innovative. Both guilds use counters so this does fit that requirement, but I don't think you'd see it at common.
Trusted Advokist 1W
Creature - Human Wizard
Whenever a permanent becomes the target of a spell or ability, add W to your mana pool.
2/2
So the white mana is designed to pay for extort costs, and I guess technically detain does target. Problems are A) the mana can be used for anything and mana production isn't white, B) open ended target triggers like this are relatively easy to abuse, e.g. this plus Shuko gives infinite mana on turn two.
Dimir and Rakdos. I always knew those crazy kids would get together
Callous Slayer 3B
Creature - Human Assassin {C}
Callous Slayer can’t block and is
unblockable.
Callous Slayer gets -1/-1 for each
card in your hand.
Unthinking. Unfeeling. Unconcerned
with the sanctity of life.
7/7
Unblockable to serve as a Cipher target and it wants (ideally) zero cards in hand to play with Hellbent.
Putting "{C}" on a card doesn't make in common. This is just too large and complex for a black common creature, and if I'm getting in for 4-7 unblockable damage, I'm not going to care much about adding cipher spells to it as well.
Pair of Thugs 2RR [C]
Sorcery
Put two 1/1 red Human Berserker creature tokens with unleash onto the battlefield.
"You can buy anything on Tin Street Alley. Weapons, drugs, even men. When you're here, everything has a price and a catch."
—Arrester Lavinia, Tenth Precinct
Unleash for Rakdos, tokens for Batallion.
Counters on tokens are a necessary evil, but imo ought not be forced to happen like this. I do like the design, how it works with the guilds, and how you refrained from giving the tokens haste as well,
Creature - Human Soldier {C}
Battalion - Whenever Recruitment Officer and at least two other creatures attack, put a 1/1 white Soldier creature token onto the battlefield tapped and attacking.
2/2
Even the Selesnya have their need for cops. They're just more subtle in their methods of selection.
Solid card and an ability I'm surprised didn't see print, though I'd expect to see it at uncommon. Doesn't really work with Selesnya, though, as you really want to be populate something more than a 1/1 non-flyer.
Inspirational Infusion 1G
Instant (C)
Target creature gets +X/+X until end of turn where X is the total amount of +1/+1 counters on permanents you control.
When you are truly part of a group, you can feel their strength pushing you into greatness.
Nice card, fits both guilds pretty well and I can see it at common. Possibly should be 2G, and the link to Gruul could be a bit stronger.
Cephalovortex 2U
Creature — Octopus Weird
Graft 1
Whenever an instant or sorcery spell you control resolves, put a +1/+1 counter on Cephalovortex.
0/0
Interesting card. I think I like it. Might have too much going on for common, but I can see it just getting under that bar. You definitely found a nice sweet spot between Izzet and Simic.
Creature - Human Rogue (C)
Whenever you cast an instant or sorcery spell, Chemister's Informant is unblockable this turn.
Might of the Righteous 2W
Sorcery (C)
Put an X/X white soldier creature token onto the battlefield, where X is equal to the life you gained this turn.