The Reformation expansion for Coup adds teams. You can't attack somebody who is in your team. You can also pay 1 coin to change teams or 2 coins to change other person's team. That money goes to a common treasury and anybody (but the Duke) can steal from it, and it can add up to quite a bit of money.
The teams add a whole new level of strategy, since you have to coup a person when you have 10 coins or more, you can effectively force that person to coup somebody in particular (because it might be the only person that's not in a particular team).
This expansion also comes with a replacement to the Ambassador: the Inquisitor, who can force another player to change a character they influence.
On top of that, the deck comes with added cards so you can play Coup with up to 10 players.