The very reliable Latino-Review are reporting the X-Men may team up with The Fantastic Four in a cross-over in 2018.
But before that can happen, the Fantastic Four reboot and its sequel (due in 2017) have to make a huge amount at the box office. X-Men: Apocalypse also needs to make a respectable amount, which is a safe bet when you look how well Days of Future Past did.
So a lot of things need to align before that can happen, but given that every single studio wants an Avengers, I would say 2018 is looking pretty good for a crossover.
The possibility alone has me excited.