To have a complete Marvel Universe crossover there would need to be four studios involved: Disney/Marvel Studios (Avengers), 20th Century Fox (X-Men and The Fantastic Four), Sony/Columbia Pictures (Spider-Man), and Universal (Namor). So, who would call the shots on the crossover movie? The easy answer would be Disney, since it brings the most successful franchise and most of the best-known characters to the table and because Disney owns Marvel Comics. However, Spider-Man is Sony's most valuable franchise and X-Men is Fox's most valuable franchise now that Star Wars is also owned by Disney. If you were in charge of Fox or Sony, would you want another company one of your major competitors, in fact having the final say on a movie that featured your most valuable franchise? Especially if that company would likely be doing the best it could to promote its own characters as the true stars over your own? Of course you wouldnt.