The concept of the film is Batman Vs. Superman with other heros. Take X-men 1 or 2, or even 3, and replace Logan & Xavier's rolls with Bruce Wayne/Batman, and make Superman into Storm & Scotts/Jeans rolls combined, and you have the screen time Batman and Superman have. The other heros are planned to be mere supporting characters, who would branch off into their own films based on popularity(aside from they see it reigniting the Batman and Superman franchises, which would be in their post-Nolan/Bale-Singer/Routh stages at this point). The plan is for Justice League to be made as the next Superman/Batman sequel after the 3rd or 4th of each series, and would introduce new actors. Like how Casino Royale opened small due to a new actor as Bond, Warners fears when Routh and Bale are done, the franchises may be done to. So, by combining "forces", the box office potential inceases a lot. An aweful lot.