Just throwing this out there, but how cool would it be, if in the middle of the next Superman film, with absolutly NO prior warning or indication, Batman just freakin' shows up, for like one scene. I know it would probably never happen, but I was just thinking how cool it could be.
I have no idea how they'd work it in either. Perhaps Wayne could be visiting Metropolis (Lois and Jimmy run off to a press conference of his). Then something goes down, perhaps Metallo attacks or something (not the main vllain) and Batman shows up to help out. After the fight, Superman tries to look through Batman's mask to discover his identiy, only to find out that it's made of lead.
Also, i'd love to see Clark Kent in the next Batman. If there's any scene at all in the movie where reporters might be present, Brandon Routh needs to be there as Clark. Perhaps Bruce could even give him a funny look as he walks by (seriously, that disguise may fool everyone else, but this is the goddamn Batman we're talking about) .