I've heard that some of the properties that are tangentially related to the X-Franchise, such as Cable or Ka-Zar, aren't actually included with Fox's film rights to X-Men, so some of the other characters that could conceivably carry a film outside of the team aren't actually available to them. For example, though it's not a solo title, Big Hero 6 is definitely an X-Men spin-off, and that is obviously not included.
Regarding the using FF villains in X-movies, I don't buy that. The only two villains you've really got to play with are Dr. Doom and Galactus, and the latter doesn't really work for the X-Men- they've fought alien menaces, but they rarely do cosmic menaces (and that's not just semantics at play). Otherwise, you've got villains that don't really make sense for the X-Men to fight (Annihilus), villains that are too tied to the FF to work (the Frightful Four), or villains that no studio exec is going to green light for a big-budget picture like this (Diablo). In fact, some of the FF villains fit 2 or more of those criteria (Paste Pot Pete, anybody?).