~ deals 3 damage to any target. Add one mana of any color.
Quick note on this card: I'm tempted to go deeper with the costs-an-extra-mana-but-adds-that-mana-back thing, because I find it to be a cool way to simulate cards that are probably too good, but I also want to show off a wider range of designs here. I know this card could be monored and just add instead of being gold and adding a mana of any color, but I wanted this card to be a little less splashable.
Counter target spell. You lose 2 life.
This one is one I know I need to be very cautious with. A 2-cmc counterspell can be dangerous in a standard meta. However, the mana cost should limit it to a handful of decks, and the life loss should make players think twice about how they use it.
Destroy target creature.
Terminate but at uncommon and without the regen text. I think standard has been ready for 2-cmc instant creature removal for a while now.
Target spell's controller may have ~ deal 4 damage to them. If they don't, counter that spell.
More 2-cmc countermagic, but this time it's a "soft" counter. Both choices are harsh for a 2-cmc instant, but I think the mana cost is fine considering your opponent gets the choice.
Purge the Living
Destroy all creatures.
I personally don't think players should have to play white to have access to a sweeper effect. This one isn't as pushed by cmc as the other designs so far, but it's pushed by accessibility. A basic sweeper effect should be available to black decks as well as white, imo.
Exile X target creatures and/or planeswalkers.
This one is sort of a top-down design harkening to a few different things. The name recalls Vraska's Contempt, which also inspired the card effect, and Jarad's Orders, which is a nod to the fact that Vraska and Jarad have both been Golgari guildleaders. The design here represents Vraska ordering a number of "hits," which her assassins carry out for her. As for the functionality of the card, it's designed more for the late game. Its flexibility in being able to target creatures and planeswalkers, possibly multiple, is why I believe this card would see as much play as Contempt or Murderous Rider.
Caught Off Guard
Target creature an opponent controls gets -4/-0 until end of turn. Up to one target creature you control fights that creature.
Just a synergistic combo of abilities from each color. As no fight spell has ever caused problems in standard, or has ever really even seen much play, I think this card is pretty safe. It's much more for limited than anything.
Create a green and white elemental creature token with "This creature's power and toughness are each equal to the number of creatures you control." It fights up to one target creature an opponent controls.
This set is a Ravnica set, if that wasn't obvious. I wanted to bring back the Elemental tokens from Voice of Resurgence, and I liked the idea of a spell that makes one fight, because of the planning and decision-making involved. I'm actually not sure about the cmc on it, because it can either be a straight kill-spell or just make a 1/1. Given the relative power level of the other cards, I figure 3-cmc sorcery is plenty safe.
Contempt of Court
Return target spell or nonland permanent to its owner's hand. Its controller can't cast spells until their next turn.
Target creature gets -3/-3 until end of turn. Tap that creature. It doesn't untap during its controller's next untap step.
~ deals 2 damage to each creature and each planeswalker and you gain 2 life.
These last three are more to try and get at least one bit of interaction into every color combo (Golgari is missing here because I already have a design I'm happy with), but I'm not quite as happy with these designs as I am with the rest.
Edit: Fixed the formatting. Trying to use these formatting tools on mobile is awful.