If Robert Downey Jr is the face of MCU, then Hugh Jackman is the poster-boy of Fox's X-Men franchise. The actor and producer was recently asked by Collider if he'd like to join The Avengers. We've previously heard promising talk from X-Men producer Lauren Shuler Donner that she'd love to see X-Men characters involved in sequels (with Scarlet Witch and Quicksilver already set for the second installment). Jackman goes further and says he's talked to the Marvel liaison to the studios about putting it together with Fox and even mentions Spidey (calling out Sony?) needing to be involved. A lot of profit sharing deals and extended contracts would have to be signed for this to even be considered. Studios aren't keen on sharing their superheroes and Marvel/Disney might not be willing to share profits with multiple studios. The film logo for Oscorp (from Amazing Spider-Man) showed up in a video game promoting Iron Man 3, which looked like the first step in bridging the multi-studio franchises into one single cohesive Marvel film universe.
"I literally asked the same question the other day to Tom from Marvel who works with all the other studios, he works with Sony and Fox, that’s his job to liaise. I said, ‘Man, can this happen?’ and he goes ‘Look, it’s not gonna be easy because you’re working with different studios and they’re their properties.’ But I believe—maybe I’m optimistic, I understand at Marvel they’ve got The Avengers, they’ve got a lot of big things going on, but at some point I just find it almost impossible that there’s not a way to bring Iron Man, all the Avengers characters, Wolverine, the X-Men characters, Spider-Man, and somehow get them in together."
"I don’t see the impediment! I’m like, okay you’ve got three studios, just split it three ways in terms of the cost, and happy days it’s all coming together! I would love to… I’m in for it, I’m totally up for it because I think that would be really exciting for fans across the board."
However, it would make sense to have other characters inhabit the same cinematic world and could be a giant way to promote other characters. Neither the X-Men films or Spider-Man films have reached a billion like Iron Man 3. The only thing is that Joss Whedon and Kevin Feige have seemed hesitant about the idea, since they could develop new characters and franchises within the MCU instead. Sharing that much screentime with outside Marvel characters might take away from others. Then again, Feige has brought up the idea of Civil War (for the third installment) which would make an excellent excuse to bring in Spidey and Wolverine. There had also been rumblings that Sony might have to sell off characters like Spider-Man and Marvel/Disney would be expected to pay top dollar to get the rights back. Fox on the other hand are about to announce an X-Force film and will likely finally greenlight Deadpool at Comic-Con.