Even if the MCU is building up to the supposed "conclusion" that is the next two Avengers movies, Marvel Studios is still planning on making movies for the next 10–20 years. Marvel has hundreds of different comic characters to introduce to the MCU, and GOTG director James Gunn has confirmed that they've been considering introducing the Richard Rider Nova.
During his Facebook Q&A Session, Gunn was asked if there were any plans to add Nova, and he said:
"Yes, definitely. Nova comes up occasionally as someone we might use… One of the things I'm doing with creating Guardians of the Galaxy Vol. 3, it will take place after the next two Avengers movies, and it will help to set up the next 10-20 years of Marvel movies. It's going to really expand the cosmic universe. We're going to be setting up new characters. It will be the last movie of this version of the Guardians of the Galaxy."
The seeds for Nova have definitely already been planted, and John C. Reilly's character in the first Guardians, Rhomann Dey, is essential to Richard Rider's origin. His character eventually becomes head of the Nova Corps and he bestows all of the power to Richard Rider after his planet was destroyed.
Though Nova may not be getting as much of the spotlight as Iron Man or Captain America, he's definitely one of the more popular heroes in the comics—even more popular than the Guardians of the Galaxy (before the movie, that is).
We have no idea when we're going to see Nova appear in an MCU movie, but I'm imagining he'll be introduced along with Adam Warlock in the next Guardians of the Galaxy movie.
No release date has been set for Vol. 3, but the movie will definitely come out after 2019.
See Also: Guardians Of The Galaxy Vol. 3's Soundtrack Will Be Inspired By Yondu This Time