5. Tonal Issues
Not only would Bruce Wayne's return betray the entire notion of him striving to retire at the end of The Dark Knight Rises, but is it really plausible that he's going to head off to help a gang of interstellar aliens and superheroes? Nolan's Batman franchise was all about gritty, stark "realism" (at least compared to most other comic book series), and that clashes completely with the fantastical aspects of Superman, Green Lantern, Wonder Woman and, well, just about everyone else in the Justice League. It would be ridiculous if viewers will have to swallow the fact that Bruce Wayne goes from fighting (comparatively) grounded foes like Scarecrow, Ra's al Ghul, The Joker and Bane to dealing with creatures from other worlds. This is why Bale really shouldn't be in it; it will create a bizarre tonal chasm that will repurpose the original trilogy in an unsavoury way and essentially see the gritty Bruce Wayne take a leap off the deep end.