Ohh F...

I think having a factions that are exclusive to each other adds a lot of replay value to a game. In Morrowind the only membership that was exclusive was the great house you picked, all the other factions you could mix and match however you saw fit and be the leader of both the church of the Tribunal and the Nine Divines for example, which simply makes no sense.

They should make it so you can only join one major faction each game.