Although we've already seen Kurt Russell's character from Guardians of the Galaxy Vol.2 from the trailers so far, we still don't know exactly what role Sylvester Stallone has in the film.
The two previously starred together in epic '90s bromance action comedy Tango and Cash, but have sadly not worked together since then. According to James Gunn, however, that's no longer the case. In an interview with Fandango, Gunn was asked straight-up about Russell and Stallone's involvement in Guardians of the Galaxy Vol.2 and where they fit into the larger Marvel Cinematic Universe, if at all.
"This is definitely not a one-off (for them). They’re both characters that we’ve taken a lot of time to develop, and they’re a big part of the MCU now," said Gunn.
As we know, Russell is playing Chris Pratt's on-screen father, but there's still no precise word on who Stallone will be playing. Rumours so far suggest that Stallone is playing one of the Ravagers; that's the band of pirates that Chris Pratt's character was working for in the first film. However, if Stallone is playing a much wider role in the Marvel Cinematic Universe, it's very unlikely that this is the case.
All the trailers we've seen so far haven't given away much in terms of plot or story beyond Kurt Russell playing Ego the Living Planet, so it's really anyone's guess at this stage.