4) X-Force
Ryan Reynolds has said a few times that he wants to bring X-Force into the franchise, and the team being formed by the end of the Deadpool sequel in order to set the stage for their (hopefully R-Rated) spinoff would be perfect.
Essentially a group of mutant assassins, X-Force do the jobs too dirty for the X-Men in order to protect their people. A long list of characters have been part of the team over the years, though Cable was responsible for forming the first iteration, and with him set to take on a leading role in the Deadpool follow-up, launching a new franchise with Nathan Summers as their leader makes perfect sense.
The Merc with a Mouth would also need to be there, of course, but if Fox is unsure whether or not an X-Force movie is a good idea, testing the water here first would be a smart business decision, especially if Cable has already put the group together when we first meet him.
The Deadpool sequel could even kick off with them travelling back in time because they need Wade’s help (good luck with that guys).