1. Martian Manhunter
2. Lobo
3. Black Canary
4. Dr. Fate
The first two are all but confirmed as the first two DLC. I'd literally be shocked if they weren't. Black Canary is a sorely needed female presence in any DC game, and she's perfect as a fighter. Fate has been talked about by Boon (mentioned in several tweets, at least), and he'd make a nice male offset to Raven as a pure caster type.
As for others, I'd like to see these the most:
5. Black Manta (awesome villain, needed for Aquaman, and cool moveset potential)
6. Starfire (another awesome female that could be more different than a Supergirl or Batgirl type, who mirror a mentor, so to speak)
7. Vixen (black female, unique powerset, was a decent presence in Justice League Unlimited show)
8. Firestorm or Blue Beetle
On top of that, I'd love to see a lot of "skin characters". For instance, if you just changed the animation for Catwoman taking her "tail" off and using it as a whip, to Cheetah flicking her hips and lashing people with the actual tail itself (and changing this in her super), you could have her as an alternate for Catwoman. Get another voice actress and you're good to go. On that note, I'd like to see people like John Stewart, Simon Baz, Guy Gardner, Supergirl, Powergirl, Zod, Ocean Master, and maybe even Metallo for Cyborg used in this capacity.
Also, hi everyone! First post here. Been creeping for a bit, finally decided to get an account.