3. Cable
Time travel is now a thing, and what better to include than Scott Summer's child from an alternate future? Cable has always been one of those characters who sometimes works with the X-Men, but usually has his own future issues to deal with. A time traveler would add a huge dimension to both the X-Universe and FF storyline, and open up huge possibilities for both. Cable has been name dropped multiple times by numerous studio heads, including Mark Millar. A character like Cable might make a great substitute for Wolverine by adding another bad-ass hero. Even more interesting, this character is the son of Cyclops, which would return the focus to the field leader of the 80's/90's X-Men. Not to mention that Cable, because he travels to the future, would help show us the future of the X-Men universe. We have the past and the present, but the future? Untapped territory. Or, alternatively, we could travel to the distant past, and explore the origins of the mutant gene. Once again, this can lead into Apocalypse territory. Yeah, I like Apocalypse...