1. The Joker
And here we come to it: the villain who should have enjoyed more of a send-off in Nolan's film universe, and who has inspired the most difficult quandry the Batman movie franchise ever faced - how to recast as successful a player as Heath Ledger. The Joker has frequently appeared in Metropolis, and given his relationship with Batman (in fact, his very definition in terms of Batman) it would make sense for Superman to require Batman's influence to take down the Joker.
Why He'd Suck The issue here is more what an appearance of the Joker in a team-up movie would mean for the character himself, rather than the impact on the movie. There's no argument that the fan-baiting inclusion of a Heath Ledger style Joker would win the film some significant positive spin, but like Luthor deserving to face off only against Superman, the Joker's next appearance demands to be in a Batman film. No ifs, no buts, the Clown Prince of Crime should not have to compromise on who he does battle with. The Joker and Batman define each other - almost as much as Joe Chill's crime against Bruce Wayne's parents define Batman - and such a high-profile reintroduction of the villain in these circumstances would be a major mistake. What do you think? Who should Superman Vs Batman face off against?