Hawkeye
And here’s the one that springs to mind the most when thinking about MCU characters who’ve had the short shrift in the movies and deserve their chance to shine. Clint Barton has long been the butt of the joke as the weakest member of the Avengers and sadly, he’s been absent from the action since 2016. He’ll return in a big way for Avengers 4, of course, but it feels like there’s still more to do with Hawkeye yet.
The difficult thing is that Barton is at the end of his career as a S.H.I.E.L.D. agent as he hung up his bow and arrow back in Avengers: Age of Ultron. The way to get around that, then, would be to have Jeremy Renner play a mentor role to Kate Bishop, a popular character from the comics who takes over from him as Hawkeye in the source material. We know that Marvel‘s building up to the Young Avengers, so Kate’s got to appear in the near future.
Though we’re imagining her as perhaps the main focus of the show, there could still be flashbacks or ghosts from his past that explore Clint’s early days and origins. Hawkeye was originally a villain in the source material, but this hasn’t really been touched on in the movies as yet. It would be interesting for Barton’s criminal days to catch up with him though if he got his own show on Disney’s streaming service.