9. Onslaught
Premise: Onslaught is a hulking supervillain who decides to erradicate Earth of both humans and mutants, so The Avengers, Fantastic Four, X-Men, Spider-Man and others are forced to band together to take him on.
Why It Wouldn't Work: For starters, it's a case of too many cooks spoiling the broth; we wouldn't get a firm handle on any one character because the film would be too busy darting around to give everyone their "moment". Plus, getting Sony, Marvel Studios and Fox to co-operate is akin to headbutting a wall until you fall into a coma. What's more, Spider-Man ends up stuck on the ground for most of the climactic fight, fending off sentinels while his more super-charged compatriots get to do all the heavy lifting.
Chance Of It Happening: 3/10