Vibe.
Damage (Grant Emerson).
Stargirl (I imagine she might return when her show hits but haven't seen her since the New52 JLA).
Jack Knight.
Vixen. She's appeared sporadically, but she needs some consistent page time and development.
The Others. Those pulp-inspired heroes who worked with Aquaman. Prisoner of War, the Operative, Y'wara.......great characters DC gave up on too soon.
Infinity Inc. Mostly just Atom Smasher, Jesse Quick, Jade, and Obsidian. The others I don't care about.
Might have some JSA members to add too, depending on who returns in Justice League. Power Girl definitely makes the list if she doesn't return with the rest.