1) Domino
If the producers really want to bring an interesting new lead into the mix in a couple of years from now, Domino is the way to go. The character has exceptional marksmanship and hand-to-hand skills, and also possesses mutant probability-altering powers which she’s used to further her career as a mercenary.
They’re basically good luck powers; if for example she jumps out of a hotel window to avoid some goons who are after her, chances are that Domino will have a safe landing in a swimming pool below. Given her connections to X-Force and Cable (she’s frequently been portrayed as his lover), common sense says that there’s a spot for her in this sequel, especially if that team and any number of its members are going to be introduced.
Seeing as Domino has a pretty antagonistic relationship with Wade, romance for them is out of the question, but seeing them work together in the comics has always been fun and would definitely work in a movie, too.