How can the next Batman bad guy be the only good guy in comics as famous as Batman? Easy! Though they're partners in the Justice League and share a passion for fighting crime and helping people, they have a lot of differences. Just look at their costumes. Superman wears bright colors looking somewhat patriotic and standing out in a crowd, while Batman hides in the shadows and tries to frighten people with his appearance. They fight fairly often in the comic books, most famously in The Dark Knight Returns, but on a number of other occasions as well. Most every time Superman comes to Gotham City, he's greeted with something along the lines of "this is my city, get out."
Pros: If done right, it would unquestionably kick ass.
Cons: It makes terrible business sense. Superman Returns was a box office failure compared to Batman Begins (they made about the same in box office receipts, but Superman cost quite a bit more to make), and The Dark Knight's success only wides the gaps between the franchises. So it could be argued that at this point putting the two heroes together could be a step down for Batman.
More importantly, should the movie be a bomb, it would damage two valuable movie franchises. A huge budget Batman vs. Superman movie that is a critical and commercial disaster would probably make audiences reluctant to see either character again anytime soon. Prior to both Batman Begins and Superman Returns, there were real plans to have the Dark Knight and the Man of Steel face off in a movie, but eventually everyone came to their senses and moved on.
How I'd do it: Superman's a reporter and is obviously aware of the goings on in Gotham City. Batman is a brilliant detective who's known to plan for any situation, so having heard of Metropolis's nigh-invulnerable hero, he's figured out not only Superman's weakness but also his identity as Clark Kent. Superman arrives in Gotham City to help out, and Batman's there to greet him with a "get out of Gotham, Clark, it's mine." Superman's freaked out at being identified, and since Batman's a wanted criminal, his instinct is to bring Batman to the police, at Batman's response is to pull out some Kryptonite. Superman retreats, and they both go off to prepare for what would have to be a 30 minute long final showdown full of destroyed Bat-vehicles and gadgets, heat vision blasts, Kryptonite-infused punches to Superman's face, and other things that would cause most comic book geeks to die from awesomeness overdose.
Casting Guesses: Brandon Routh wasn't bad in Superman Returns, but he wasn't great either. Superman is almost impossible to cast with established stars, so I'd say go with an unknown.
Verdict: It may happen someday, but not anytime soon, and, I'm sad to say, certainly not in the next Batman movie.
After The Scarecrow, Ra's Al Ghul, The Joker, and Two-Face, who should be Batman's next enemy in the Batman Begins/The Dark Knight series? Well, as of now we know that Tom Hardy will play Bane and Anne Hathaway will play Catwoman.
Check out the master list for all the villains covered. I've pretty much run out by now. Obviously, there are more Batman villains, but not any of significance that I know of. As of February 2009, no one is confirmed as a villain. Not the Johnny Depp Riddler, not Phillip Seymour Hoffman Penguin, nobody.