I think it would be cool if ingame you could make unbreakable and breakable alliances, basicly you and one or more players can make a alliance, if its unbreakable players can never attack eachother and when the game ends, the players in that alliance win, in breakable alliances, players can attack eachother and break a alliance, and when its just them at the end of the game, the alliance breaks automatically and they can fight eachother freely. This would support teamwork, and be p. usefull in the big games so I think its a good idea.
Edit :
I guess to balance unbreakable alliances, only 4-3 players should be able to get in one, so the rest will have a chance.