Have a feeling that's gonna be Cable's role. Basically been his job in the Cable/Deadpool run, and he's really the only one to talk sense to Nena and Wade. You kind of have to be crazy to team up with these guys.
If there is a more level headed type of role (whos not the lead) I'd be worried they wont be able to keep up with everyone else and seem out of place. I guess Cannonball would fit that bill pretty well if they needed a more level headed character in the crew.
Fantomex is as far from a straight laced character as you can get. Dude has three brains and cant be trusted by anyone under any circumstances. Even Psylocke cant trust him. He's just as likely to back stab as Deapdool is. If not more. Thats what makes him so interesting, and why he fit perfect on Remenders UXF. Everyone on that team has issues beyond repair. Im all for his addition but I dont think he's a good option as the more normal straight laced of the group. Dudes all mystery and illusion down to his powers and should have the audience always guessing what his true intentions are.