There shouldn't be such a thing as an "X-Men" spinoff, just because none of the X-Men actors are lead role material. Wolverine a cool character, but doesn't work outside of the collective. There's also a Magneto spinoff in the works, which would subsequently bomb, just because he's a rather boring character.
That's like suggesting a "LOTR" spinoff with Frodo only. No sense in that, because it would be filled with other characters anyway.