Problem is that he's pretty damn limited in terms of the comic storylines he can have, given that Fox only has X-Men and FF-specific characters to work with. I'm assuming Mephisto is with Marvel Studios, certainly most of the other "cosmic" characters are. You could still make a good, original film of course. And I agree that obscenely powerful cosmic creature who can fly around in space makes for a more interesting potential movie than lame Louisiana stereotype guy.
I think the X-Men franchise has plenty enough to do without needing to worry about a cross-over gimmick. But if they want to do it, I'm for it as long as it works. It would be a lot more interesting if they took a more appropriate tone with the FF though, as the media darling explorers of the FF could then have a more interesting mix with the ever-hated, frequently dark, and sometimes pessimistic tone of the X-Men. Two groups with totally different motivations and experiences with the public, the government, their villains, etc. The way things look so far though, the FF could easily fit in as just another handful of X-Men.