The X-Men franchise started off as a standard series, one movie after another. Then the Wolverine spinoffs took some minor detours, and with last year's Deadpool release, now 20th Century Fox essentially has their own superhero cinematic universe. However, both Deadpool and the upcoming Logan feel much different from the other X-Men movies, and with the former paving the way for an eventual X-Force movie, it begs the question of if that project will feel more at home with the mutant superheroes or in the Merc with the Mouth's corner of this world. According to writers Rhett Reese and Paul Wernick, it sounds like X-Force will follow more in Deadpool's footsteps tonally.
Rhett Reese acknowledged to THR that the superhero cinematic universes have their own unique tones, and the X-Men world has generally been somewhere in the middle of Marvel and DC. Deadpool, however, found a new tone for the franchise, and X-Force will keep that lighthearted, but still edgy, feel. Reese explained:
I think what we stumbled into was a new tone, and I haven't seen Logan, so it's tough to say if they have it, but I think we hope to have our own universe that is defined less by characters and timelines and things like that and more by tone. The hope is Deadpool 2 and X-Force and future movies all be this new, consistent, sillier tone. More self-aware tone. And edgier and rated-R tone. We want to be establishing the universe but also focusing on each individual movie and not worrying too much about building a larger threat to the world or a larger plot machination.
Considering how involved Rhett Reese and Paul Wernick are in getting the X-Force set up in this universe, it's not too surprising that they want it to resemble Deadpool more than the X-Men installments. Plus, in the comics, the X-Force is a blacks ops team of mutants that is more willing to take brutal action against any enemies against mutantkind. Extra violence is called for. The "sillier" tone will be harder to pull off if Deadpool isn't part of the team, but like Reese said, the goal with X-Force isn't to merely be another cog in a giant machine. They want the militaristic group to help expand the X-Men universe, but they also want them to lead an enjoyable, self-contained adventure; one that's packed with plenty of action, but is also self-referential and humorous at times. That helps make it more of a unique product.