We'll start here by saying this is far from a done deal and more just about conversations that Adam McKay has been having lately. But speaking to the Happy Sad Confused podcast, on the occasion of his excellent Dick Cheney biopic Vice's release, McKay revealed that he's been having ongoing discussions with Kevin Feige and Marvel Studios about possible projects for some time, and that the third Guardians Of The Galaxy might be something he'd find difficult to turn down.
Specifically, McKay says the character of Nova){href='https://en.wikipedia.org/wiki/Nova_(Richard_Rider)' target='_blank' rel='noopener noreferrer'}, head of intergalactic police force the Nova Corps, is one he'd be particularly interested in working with, and one that's likely to introduced in Guardians Vol. 3 (news already confirmed by Feige).
McKay's relationship with Marvel Studios dates back to the original Ant-Man. After Edgar Wright's departure from that film, he declined an offer to direct it himself (because of his friendship with Wright), but did help with the re-write process out of a desire to be "involved somehow". The director's chair for Guardians Vol. 3 is, of course, currently similarly vacant thanks to the unfortunate firing of _Vol_s 1 and 2 director James Gunn.
Since Ant-Man McKay says he also discussed working on an Inhumans project (presumably before it became the short-lived TV series). But he says, even more than a Nova-focused Guardians Vol. 3, the one he really wants is a Silver Surfer movie – which he'd approach in a manner somewhat akin to the Wachowskis' Speed Racer...
It seems like only a matter of time before a Marvel McKay movie happens. Watch this space for further developments.