"I think that exists more in the Guardians universe," Russo told ComicBook.com in regard to Nova. "That would have to be a James Gunn thing. But again, we just had too many characters on our plate. And you can't introduce somebody who doesn't have their own movie like [Captain] Marvel. You can't just throw them into the final chapter of a book."
The Richard Rider incarnation of Nova is a member of the intergalactic police force known as the Nova Corps. Created by writer Marv Wolfman and artist John Romita Sr., he first appeared in The Man Called Nova #1 in 1976.
With Nova being more of a cosmic character, it does make sense to introduce him in a Guardians of the Galaxy film. In fact, the Nova Corps has already appeared in a Guardians film, so we at least know about its existence in the MCU. However, given the fate of Xandar in Infinity War, the group's future remains unknown.
Nevertheless, while James Gunn hasn't spoken about any plans to utilize Nova in the franchise, there are rumors of a Nova solo film. Another comic favorite, Adam Warlock, was teased at the end of Guardians of the Galaxy Vol. 2.
Written and directed by James Gunn, Guardians of the Galaxy Vol. 3 does not currently have a release date.