Actor Vin Diesel won the role of Groot in Guardians of the Galaxy after months of teasing fans about meeting with Marvel, and fans responding with pleas to make him part of the Marvel Cinematic Universe. Normally playing a key role in the biggest Marvel movie of 2014 would clear Diesel from rumors of playing any other roles in the studio's long list of upcoming films, but since he only provided the voice for Groot there's still the possibility that Marvel could bring him back for a live action role.
In fact, the original intention was for Diesel to play a role in a Phase Three movie that would be a "fresh new IP," set for release in 2016 or 2017, and Diesel has said that he played Groot to appease fans who "wanted something a little more immediate." Shortly after Guardians of the Galaxy hit theaters, Diesel returned to teasing a future role in the over-arching franchise with the coy statement, "I get the strange feeling that Marvel thinks I'm Inhuman," an obvious nod to Marvel's Inhumans, which is set for release in 2018.
Diesel's hints of involvement in Inhumans got a lot less coy over the weekend, when he posted a photoshop of himself wearing a shirt with the Inhumans logo with a backdrop asking "Are you Inhuman?" This was probably made by a fan, since Diesel regularly posts fan art of himself, but the fact that he posted it is further evidence that he is in talks with Marvel for an Inhumans role.
In the comic books, the Inhumans are the result of genetic experimentation by the Kree (who have already been seen in Guardians of the Galaxy and Agents of S.H.I.E.L.D.) on primitive humans from Earth. After the Kree abandoned the subjects of their experiments, the Inhumans formed a society of their own that was comprised of human-like beings with extraordinary abilities, the strength of which decides their place in society. At the top of the food chain is the Royal Family, which is led by the king of the Inhumans, Black Bolt.
Obviously there are still a couple of years left to go before Inhumans will even start filming and nothing is set in stone yet, but everything we've seen so far indicates that Marvel has been aggressively pursuing Diesel for a Phase Three role, and the actor has been strongly suggesting that it's a role in Inhumans. Whether that's true or whether he's simply trying to throw people off track remains to be seen.
Inhumans is set for release on November 2nd, 2018.
Source: Vin Diesel