Much as I'm excited to see a glut of super heroes... Superman and Batman being together is such a great concept on its own that I'd like an entire movie dedicated to that before moving on to the others.
I think that's still likely to be what the
bulk of the movie is about.
And then towards the end, as they're about to face down whatever huge threat there is, Batman will decide to enlist the help of others that he's been keeping tabs on-- presumably because Superman will have been severely weakened at that point and is not at full strength (because how ELSE to explain the need for other superheroes when someone with his powers is around? Lol)