He's the Merc with a Mouth, but he isn't one of Earth's Mightiest Heroes, apparently.
Deadpool himself, Ryan Reynolds, posted an image on Instagram of a "rejection letter" from Tony Stark. In the letter with the subject line "Joining the Avengers," the armored Avenger bluntly tells Deadpool that he can't join the Avengers, and he suggests that the anti-hero with a healing factor should instead go bother Professor X. On the Instagram post, Reynolds also congratulates Avengers: Infinity War on its massive opening weekend. Don't feel bad for Deadpool, though. He'll form the first cinematic version of another superhero team, X-Force, in Deadpool 2.
While Disney's acquisition of Fox's key assets is expected to be done by Summer 2019, Marvel Studios President Kevin Feige has implied that it may be awhile before the X-Men (and potentially Deadpool) could appear in the Marvel Cinematic Universe. Whether or not the current X-Men universe actors will reprise their roles or if their characters will be recast and reinvented in the MCU has yet to be revealed.
Debuting May 18, director David Leitch’s Deadpool 2 stars Ryan Reynolds as the titular Deadpool, Morena Baccarin as Vanessa, T.J. Miller as Weasel, Leslie Uggams as Blind Al, Brianna Hildebrand as Negasonic Teenage Warhead, Stefan Kapičić as Colossus, Zazie Beetz as Domino, Julian Dennison as Russell and Josh Brolin as Cable.