WB announced today at comic con that this will come out in 2015. Will be the follow-up to MoS! Nolan will produce this. Cavill will return as Supes and a new Batman/Bruce will be cast. http://herocomplex.latimes.com/movies/comic-con-2013-superman-batman-movie-will-follow-man-of-steel/#/0 Spoiler
Spoiler If this is a direct sequel to Superman, then I think the story might be set. Batman goes to check out Metropolis after the destruction of it. I imagine he'll be looking to see if he can trust in Superman or not. Probably won't be able to in the beginning, they'll have an epic set piece (although an interesting problem of how a human can fight a superpowered alien). Meanwhile Lex is coming out as a public figure and trying to get the public against Supes. Plot happens, Batman notices Luthor is bad and Supes is Good, they team up to fight against either Lex or a minion of Lex. regardless, I'm pretty excited.
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Who would be a better choice not named Nolan though? I'm also hoping for a new director for this team up movie or at least have Nolan be more involved.
A quote that was read during the WB presentation that will help us understand what the movie will be about. From the transcript of the presentation http://www.firstshowing.net/2013/comic-con-2013-live-warner-bros-extended-hall-h-presentation/ The quote is actually from The Dark Knight Returns I believe which does feature an older Batman.