Omg that's a hard one. I think there are a few characters in Marvel that can get some shine. For instance Black Widow is in the Avengers movies, great but She should have a serious series about who she is and all those missions she has taken.. I believe is an Anti Hero wrapped up to be a hero and she is totally not. She walks that line..
The illuminati : make it dark.. These are supposed to be heros that pull the strings behind the scenes in different events that occur in Marvel.
Something cosmic like the Dark Phoenix force..
Blade which they tried before and failed but with Netflix maybe there can be a chance!
Psylocke also has a very interesting story too