I thought about discussing this in The Complex thread, but I think for something like this, a separate thread would work better. If the mods feel different, I apologize. I still think this is worth discussion, even if it is wild speculation. Then again, this is a message board. Wild speculation is kind of what we do.
I came across this article on MoviePilot recently, which came out on August 1st. It talks about the possibility of the fourth Avengers movie involving the X-men through a similar Sony-type deal. Now this is wild speculation. MoviePilot is NOT a major entertainment news site. However, I do think there's reason to believe that such a crossover could happen and it might actually be viable now, even if it is extremely unlikely.
This past SDCC had a pretty big moment between Marvel and Fox. We saw in the Cup O' Joe Panel how Marvel Entertainment is now working closely with Fox on Legion and another X-men TV show. That sort of coordination is pretty unprecedented, considering how Marvel has undermined its Fox-associated properties in recent years. This may be the first bridge to greater collaboration. We already saw that Marvel is willing to collaborate with Spider-Man. That's why they made the Sony deal. Now, they're coordinating with Fox on Legion. That doesn't change the current situation with the movie rights, but it does create an opportunity, one that could be lucrative for both sides.
In addition to Legion, Marvel announced recently that Avengers: Infinity War would not be broken up into two parts. Only the third Avengers movie would be called "Infinity War." At the moment, the fourth Avenger movie set for 2019 has no official title. If I had to speculate, it may even be pushed back a bit, just as the Inhumans movie was pushed back. By this time, Hugh Jackman will have likely retired from playing Wolverine. Ryan Reynolds would have done two Deadpool movies thus far. A Gambit and New Mutants movie will have likely come out. And we have no idea whether there will be a follow-up to X-men Apocalypse so I'd say the X-men's movie plans are tentative at best.
If the X-men and Avengers are going to cross over, this is the best time to do it, after Infinity War. I think that a year ago, this was impossible. Then, I think the shake-up with Marvel Sudios changed things. When Kevin Feige, who has close personal ties to Fox (there was even an Easter Egg mentioning him in the Deadpool movie), cut ties with Marvel Entertainment, he separated himself from the politics of Marvel. That means he can negotiate closer with Fox if he wants to and the powers that be in Marvel can't stop him. They already made a deal for Spider-Man. He's coming to the MCU in 2017 and if that movie is successful, it will set yet another precedent. Coordinating with other studios can work and it can be profitable. If there's a larger profit to be made, why not pursue something with Fox?
It may also be the case that now that Marvel Entertainment can't influence or coordinate with Marvel Studios as easily, they're turning their attention to Fox. With Fox, they could have indirect influence of sorts. We're already seeing that manifest with Legion. We may see more once more info about the other X-men show comes out. So the barriers that kept the two apart could be fading. And when the time comes to follow up Infinity War, what better way to raise the stakes than to bring the X-men into the mix?
So does that mean we'll get Avengers vs. X-men: The Movie? It's possible, but not very probable in my mind. I still think that Marvel and their Disney cohorts aren't going to go that far with Fox. They'll coordinate on TV shows. They'll gladly take a share of the profits from those shows, bearing in mind that Fox pays to produce them. I just don't think either side will be able to negotiate the right kind of deal to make this work. I'd love to see it. It would definitely open up whole new avenues for both franchises to explore. It just may be too big for the current politics between the two companies. But it's an interesting thought to say the least.