I don't think X-Force should be more than a singular story for Deadpool. I think if you make it a separate movie series focusing on the group, you threaten to dilute Deadpool into a supporting character, removing the public's interest in him as a singular hero.
Deadpool needs to remain central to the theme and world build through him. If Fox starts making other movies that resemble the humor and edginess of Deadpool, you lose the uniqueness of that series.
Deadpool remains that one universe where you can pull in Marvel characters in a more adult fashion. If I were Reynolds and Fox, I would covet that position instead of thin it out.
It's hard to remain unique in this ever expanding universe of hero films. Deadpool still retains that edge. They need to fight to hold on to what makes them unique.
