Warner Bros. released today the official synopsis of the upcoming Batman v Superman, and it hints at a few things and more or less lays out the entirety of the film. I wouldn't read any further if you're sensitive to spoilers.
Fearing the actions of a god-like super hero left unchecked, Gotham City’s own formidable, forceful vigilante takes on Metropolis’s most revered, modern-day savior, while the world wrestles with what sort of hero it really needs. And with Batman and Superman at war with one another, a new threat quickly arises, putting mankind in greater danger than it’s ever known before.
I'm going to go out on a limb and say that by the end of the film the two heroes will have to work together to save the planet from this new threat (Doomsday?) and will decide that they will probably need even more help in the future. A kind of League, if you will. Of Justice.
Or they could just be talking about Lex Luthor, but would it really take both of those men to take down a single human man? Luthor is formidable, but it really does sound like they're teasing something a bit more interesting.
It's an interesting way to go with the film, especially since the first trailer was so clearly seen from Batman's point of view, looking at Superman as if here were a powerful, unknown threat to the world.