Fans have known for a while that action hero Sylvester Stallone will be appearing in James Gunn's Guardians of the Galaxy Vol. 2. Though we may not have an idea who he'll be playing, it's possible that his character had been leaked online.
According to MoviesBook, Stallone's role was revealed via an international press release breaking down all of the cast members and their roles. Kurt Russell was credited as Ego the Living Planet, but Sly had been cast as a character named Stakar.
Stakar in the comics is better known as Starhawk, and his character is proved to have as long a lifespan just like Russell's Ego — that is, if you count being constantly reborn in your infant body on the same boat as living forever.
It had been confirmed that Sly and Smallville's Michael Rosenbaum had been cast in some "key roles" in the Marvel Universe, and having Stallone be Starhawk would make sense. My best guess is that Stallone, Rosenbaum, and Russell have characters that form some sort of council, what with their longevity, and they will serve as some kind of replacement for Marvel character The Watcher. Then again, that's just me guessing.
Guardians of the Galaxy Vol. 2 comes out on May 5.
See Also: Sylvester Stallone And Kurt Russell Could Return For Future MCU Movies