People have been enamored with David Harbour ever since the show came out. You may not have realized it, but he is one of those actors who has been around for a long time. The show gave him a huge boost, and it only grew after he was cast in Black Widow.
It was only a matter of time before we saw him again in a film like Yelena Belova because of how fans reacted to Guardian. In September of this year, we were told that Guardian will be in the main team for the upcoming movie along with Yelena, Sebastian Stan's Winter Soldier, and Julia Louis-Dreyfuss' Contessa.
While that film isn't out for another few years, io9's Germain Lussier recently got a chance to speak with Harbour about it. Though Harbour admitted that he hadn't been given a script yet, he did say that he was told how the movie would work. The idea is intriguing.
Guardian's inclusion on the team is odd since he hasn't done anything evil or malicious towards the franchise heroes. Harbour told io9 that the strangeness of it all is part of the point.