Alliance Card / Game mode
A game mode option or card that allows players to actually form teams/alliances in game in which they would have to break the alliance in order to attack that player again. After breaking the alliance they would have to wait X number of turns (can be customized in the options). When in an alliance they will have the team chat option and will be able to transfer armies and things just like a normal team game.
The Diplomacy Card already exits and there are several diplomacy mods