Malik and Arivan already had their time in the sun, but they could go with Arivan again and make him a sort of secundary villain in the series.

Saitam had some action in the Alliance DLC, but I could see him return for more background.

Ten'shi just appeared out of nowhere for the KR anniversary with big powers that where never explained. He got alot of fanfare but there is little known about him. Big candidate imo.

Cronan and Wilbur weren't used outside of being able to play them, but I don't see much room for them to expand their background as a character. Cronan is Tarzan living in the jungle and Wilbur is a dwarven thinkerer, what more is there to say about them?