Definitely Nathan Summers (Cable) should be leader of the team.
Would be good to se Stephen Lang in this role
but if he's too old, and not enough good actor to play main role of the movie, then maybe Gerard Butler
As for the team, we already have in DOFP:
Boo Boo isn't enough high (180cm) to play Warpath, but let remember Warpath wasn't giant since begin of his first appearance. He was average height in Hellions, he rose later. Boo Boo's father is 194cm height, men rise until 24 y o, Boo Boo is 19 y o, let's hope he will rise 10+ cm in next few years. If yes, and he will bulk up, then he'll be ideal Warpath.
What about other X-Force members? IMO would be perfect for random selection, choose characters from different X-Force rosters.
For sure, there has to be Cable's love Domino. I don't know which actress, but should be famous for this role, maybe Keira Knightley, she played in movie ,,Domino" 8 years ago.
Definitely Deadpool, because we won't see his solo movie. And only Ryan Reynolds in this role, he read comics and really wants to play this character.
There has to be my favorite female mutant Psylocke, but again Mei Melancon,
or if this is alternative timeline so give her in british body, or just recast?
That would be solid team, but would love to see not only original members from X-Force comic, maybe add Bishop. I like this character, and in DOFP he won't have enough time at big screen. Would be good to see this badboy in this badass team. Omar Sy has a potential, and I want to see him in this movie.
Maybe Archangel? But we have to see first Angel to Archangel transformation, made by Apocalypse. That would be reason for Warren and Nathan to cooperate, take revenge on Apocalypse. Talking about villain, before final battle against Apocalypse (best together in crossover with X-Men), they should fight vs Mr. Sinister and Stryfe.
IMO Billy Zane was ideal for Essex 10 years ago
now his bald, but remeber Grammar was bald too. Zane is ideal for villain character.
Dwayne Johnson should be Apocalypse