We're talking new characters here, not returning roster. I'd personally like to see a representative of all the Lantern Corps. or at least Atrocitus, Larfleeze and Black Hand/Nekron, but just make sure they have varied move sets otherwise we'd see some lantern saturation. Red Hood was a popular choice that people kept asking for in the first game, but I think any more Batman characters would be too much. Starfire and Black Canary seem like they would be good additions, especially if they add tag battles like in Mortal Kombat (imagine Raven/Starfire and Green Arrow/Black Canary team-ups). I'd love to see Captain Atom too even if he was killed off in the comics. Can't really go wrong with Darkseid either, maybe as the final boss as well, or just rip off Marvel vs. Capcom 3 and have it be a giant Anti-Monitor fight.