John “Thunderbird” Proudstar was chosen by Professor Charles Xavier as a member of the second generation of X-Men, but he died shortly after, becoming the first member of the mutant team to fall in the line of duty. His legacy has lived on though, including through multiple returns (none of which have lasted surprisingly) and his son. We actually met Warpath in X-Men: Days of Future Past‘s future set scenes, so what ultimately leads him to the X-Men could very well be explored here.
Born into an Apache tribe, John served as a Marine, but his anger and abrasiveness alienated him from his fellow X-Men and may very well have played a role in why he died. Throwing a character like this into the mix would change the usual team dynamic in a big way, while the fact that he’s extremely powerful – Thunderbird has super strength, extreme durability, and a very impressive set of fighting skills – also brings something different to the table.