Speaking with Yahoo Movies, four-time X-Men director Bryan Singer has confirmed that there are plans afoot for a crossover with Fantastic Four. “Those ideas are in play,” he says. “That would be a natural match-up because they’re both ensemble films and there is a natural mechanism by which to do it.” Since the new Fantastic Four movie is set to deal with interdimensional travel, it's safe to assume this is the "natural mechanism" he's speaking of.
"It deals with time," Singer finishes. "That's all I'm going to say."