No matter how many times the conversation happens on the Internet, there's very little chance Fox and Marvel will ever get to the table to discuss the X-Men joining the MCU. The Fantastic Four might be a more likely option, but the reason for that is exactly the same reason X-Men won't - it's all about success.
Fox might need the assistance from Marvel for their first family, but the X-Men franchise has been financially very lucrative for Fox and though they were very quiet at CinemaCon, there are plans for at least five new films - X-Men: New Mutants, Supernova, Deadpool 2, X-Force and Gambit. Nothing is going to change on that front for a while.
It probably also helps that Fox paid Marvel a paltry figure to get the rights in the first place.
According to a transcript from the Industry Focus podcast on The Motley Fool, the 1993 deal involved only $2.6million - just over 1% of the production budget of X-Men: Apocalypse.
When you consider that Deadpool made $322.2m in profit, it's no wonder they'd be resistant to passing the rights back for anything. And while Marvel did get a slice of ticket sales (estimated at 2% for the original X-Men), that's still a tiny drop in the ocean, even on the likes of Deadpool.
That sort of business shouldn't come as much of a surprise: Marvel didn't have their rights in order before the idea for the MCU came about - which is why they're still so messed up for certain characters - and it's ridiculous to think on the missed financial opportunities they passed on.