If there's one thing that would be guaranteed to cause comic book fans to lose their s@&t and weep tears of uncontrolable joy (apart from getting a footmassage from Loki, of course), it would be if Spider-Man, the X-Men and the Avengers all appeared in the same movie.
Marvel Studio chief addressed this issue at a Q & A, and while he didn't rule out the crossover ever happening, he did warn that it won't happen anytime soon:
[The Falcon] is a good example of the amount of characters that Marvel already has, that we can add to future installments of the films. So I would say not anytime soon. Would we be in a position to team up with other studios that have a few of our characters, because we have so many characters ourselves? But I would never say never.
If it ever happened, it would probably be an event so stratospherically awesome that it would cause the collapse of our sun and the destruction of our solar system. This is one reason why it will probably never happen, but the other, more prosaic reason, is rights. Spidey is tied up over at Sony, the X-Men are owned by Twentieth Century Fox and The Avengers are in bed with Disney/Marvel. But with The Avengers raking in over $1.5 billion, the money-making potential for a super-duper superhero ensemble is huge.
Would you like to see a superhero crossover movie or should the properties remain separate?