Mark Millar, the Kick-Ass creator who serves as a consultant to 20th Century Fox on their Marvel Comics properties, told SFX Magazine that the X-Men and Fantastic Four films he's producing will take place in the same universe and unambiguously implied that the two families of films could cross over.
"Without question I think you have to see some of these guys showing up in each other’s movies," Millar told the magazine when asked directly whether Fantastic Four and the X-Men franchise would interact. "I think the most exciting thing in superhero movies, until The Avengers came along, was when Nick Fury showed up in Iron Man. Even though it was a guy with an eye patch it was really cool – and I expect we will see more of that."
The quote alone doesn't confirm the shared universe without context, but given the content of the question, Millar's meaning is clear.
The writer also noted that while the first two Fantastic Four films were not particularly great, he still has warm feelings toward them, and that the fans deserve a definitive take on the characters, as has been provided for characters like Spider-Man and Batman.
"These first two movies were not very good but I’m quite sentimental towards them," Millar said. "It sounds obvious and it is. Sam Raimi gave us a great Spider-Man. Christopher Nolan gave us a great Batman. So a brilliant director is probably the most important thing for a great Fantastic Four. Tim Story was seen as an odd choice for Fantastic Four but when you have a Josh Trank you know you are in safe hands."
Millar is partially responsible for the decision to go forward with X-Force, an expansion of the X-Men universe directed by Kick-Ass 2 director Jeff Wadlow.