I think the cat's out of the bag by now but SPOILER WARNING (?) if you haven't seen X-Men: Days of Future Past yet:
Kelsey Grammer – along with just about everyone else who's ever been in a an X-Men movie – appeared briefly in the latest installment of Fox's franchise. And he's like one more crack at it.
Speaking to Collider, Grammer says it was actually his decision to return for Days of Future Past, and he sought out Bryan Singer to make sure it happened:
Actually I called Bryan. I bumped into Hugh Jackman and he said, 'Oh you're gonna be in another X-Men, mate!' and I said, 'No I'm not, I don't know anything about it.' So I made a couple of inquiries, got a hold of the script, found out that it's primarily dealing with the past and that timeline I don't have a place in, but I saw that at the end there was this coda where he made an appearance. I said, 'Listen, I really wanna be involved,' so Bryan arranged for it and I had a lovely time.
And he goes on to reiterate that he'd love to put on the fuzzy blue costume again, presumably for X-Men: Apocalypse:
I hope to do another. I hope they find some way to come up with a new story that involves Beast in my timeline.
Apocalypse, as far as we know, will mostly involve Beast in his younger, Nicholas Hoult form, and will take place in the 1980s for the most part.