Cable, Domino, Deadpool all of them will be X-Force members. And all of them will be featured in Deadpool sequel, and we will get nice character introduction and development from them for X-Force movie.
Next is Warpath. He was original member of X-Force, he was in DoFP in small role, he was featured in original X-Force scrapped script (which I still think will serve as baseline for new script). All together he makes most sense. Original scrapped script had 5 team members. Cable, Domino, Warpath, Cannonball, and an unknown female character (which I think it's Rogue). Cannonball will be in new Mutants so he is gone, likley Deadpool taking his place. That leaves "unknown female character".
Rogue as "unknown female character" makes most sense. Giving Gambit will be featured in present day with Deadpool and X-Force, she makes most sense giving he will eventually join them. More than Psylocke and maybe even more than Feral. I could see maybe even Colossus part of it if Fox want to use him again on big-screen. There is not much choice for him to appear in present day as part of group.
So this is my X-Force 5 members team: Cable, Domino, Warpath, Deadpool, Rogue. Eventually to join Gambit and maybe Feral or Colossus. I would personally want Psylocke over Feral, but unless Cable goes in 80's and bring her in present day that's not happening.