It's kind of a long shot, but I'd say Norman Osborn. Yes, the Green Goblin. Again.
Assuming they don't just kill him off at the end of whatever movie he's in, I think he has a lot of potential. IIRC, in the comics his Norman persona is into politics. Doesn't he even become president at some point? To Spider-Man he's the classic glider-flying maniac, but to the Avengers as a whole, he could become a problem that can't just be punched in the face.
Maybe build up to the Siege storyline after Thanos? Probably not, but it's an idea.