Whatever happens in Avengers: Infinity War and its untitled sequel Avengers 4, it's at least a safe bet that Thanos will leave devastation in his wake. After these two event movies, the shape of the Marvel Cinematic Universe will be altered, its foundations shaken, part of its cast retired and its future... well, that's a mystery. We know that the MCU will press on, however, that a new Spider-Man film will be coming, that there will be another Black Panther adventure, and that the Guardians of the Galaxy will return for a third volume.
We also know that much of the future of the MCU lies in the stars. Late last year, Marvel Studios president Kevin Feige revealed that beyond the next two Avengers films, he would like to see more superheroes have adventures in outer space. If the next crop of MCU movies is set to leave their Earth roots to embrace the endless reaches of the cosmos, then it would make perfect sense to introduce a new character to shepherd this new age of Marvel: Richard Rider, aka The Human Rocket, aka Nova.
Richard Rider is a fan-favorite character that many have waited a long time to see appear on-screen. From the start, Guardians of the Galaxy director James Gunn has been fielding inquiries as to whether we would ever see the Human Rocket fight alongside Star-Lord, Gamora, Rocket and the rest of the team. While he appeared hesitant at first, the director eventually warmed up to the idea, stating that Nova could join the MCU at some point in the future. Whether it actually happens in the third Guardians film or not, the director did reveal that the threequel would be a vehicle showcasing "the end of this iteration of the Guardians."
If we're to take Gunn at his word, it means that Vol. 3 could be the last space adventure for Chris Pratt's Peter Quill. Whether that entails the character's death will be up to the writer/director (and Pratt's contract, to be honest), but it does mean that he intends for the team to live on in some capacity -- if not featuring all of the founding members, then their idea, their name and their spirit. That means that the door would be left open for a new space-bound human to protect the galaxy, and who better to fit that bill than Richard Rider?