They better use the same characters and actors already signed. It would be way to go, connecting movies and developing characters just introduced.
When we already have Warpath, Sunspot and Bishop signed... we dont need a whole different cast for X-Force, you know
just adding Cable and Domino, they would have a seriously BAD-ASS team.
If they want another female, they can introduce Psylocke too, or use Blink again, and thats it.