The final lines of the Batman/Superman movie:
"Bruce, even between the two of us, we can't face this!"
"You're right, that's why I've recruited help."
Cue splash scene of 4 more heroes arriving on the battle scene.
Continued in..... JUSTICE LEAGUE.
Rock ballad, credits roll.
There's the problem with that setup: no introduction to those 4 more heroes at all. And that's how this would fail, and why The Avengers was a success. Each of the heroes already had his/her intro in previous movies, and the audiences liked them.