I'd have to go with Spider-Woman. With her supporting cast, allies, rogues gallery and connections to already established MCU continuity (Hydra, S.H.I.E.L.D.), she'd have the most potential. As Dark raven pointed she's also connected to the supernatural side of the Marvel universe and if they bring in High Evolutionary with Scarlet Witch and Quicksilver, there's another connection. She would be the easiest character to bring in.
Ms. Marvel and She-Hulk would be awesome too.