6. The Legion of Doom/Injustice Gang/ Secret Society of Super-Villains
Basically all of these are mass super-villain team-ups that would end up getting all the heroes to join together as well. While I would love a moment with the heroes seeing a horde of their enemies charging towards them on film, there is a problem. If the film has to induce the line-up of the league that could take time, even if they realise that probably everybody knows who Superman and Batman are. Now, imagine having to do the same for a group of super-villains when Lex Luthor and the Joker are the only two you can get away with skipping the establishing details for. The film will probably be two hours, but it will be difficult to spend the first hour having to introduce everybody without getting dull.