These are characters I think that deserve to be regulars and fit with the show.
1: Amadeus Cho
Originally a Hulk supporting character, in the same vein as Rick Jones, Amadeus is actually a pretty well rounded and awesome character in his own right. He's one of the 8 smartest people in the world........... And he's a teenage high school dropout. He's also been close friends with Hercules, even eventually gaining Herc's title as Prince of Power and being given his magical Golden Mace. He also occasionally goes by the codename Mastermind Excello.
2: Rick Jones
What's so special about Rick Jones? Well without him we wouldn't have The Hulk, and has been his most famous supporting character. He's also been a crime fighting partner to both Captain America and Captain Mar-Vell, is an honorary member of The Avengers, and as of 2008 has become the intelligent blue Hulk-like creature known as A-Bomb though he retains his full intelligence
3: Rikki Barnes
Originally from the Heroes Reborn universe, and the Bucky of that universe as well, she has since folded over into the main Marvel universe and has taken the name Nomad which was a name Steve Rogers took after becoming disillusioned with America for a time. For the show she could easily be the original Bucky's grand-niece, sharing the family nickname.