I would die for online co-op...I really loved Xmen Legends.
I actually don't care if they put co-op in the next one, at least not for more than two players.
I'd like no more levels where you only have one or two people, as those kind of broke up the flow of the game to me.
I always want the ability to play through the game again with my already-leveled characters at higher difficulty levels.
If they do that last thing, I'm in. If not, I'll have to pass on the next one. I've already lost interest with the first one, and a little over a week and a half is not quite satisfactory for me.
Don't get me wrong, it's a great game. It just needs more replay value for singleplayer.