Martian Manhunter
Black Canary/ Zatana
Atrocitus/ Zoom ( Boon asked about him once)
Darkseid
This is would be a great list. I'm sorry but Starfire, Beast Boy and Swamp Thing should NOT be in it! We get two heroes and two villains so we let's be practical. There is no way for there be both Beast Boy and Starfire. Swamp Thing over Black Canary/ Zatana or Dr. Fate would be a travesty. Darkseid because he is freaking Darkseid, I shouldn't even have to justify him. The last villain is a toss up, depends on who the second hero is. If the second hero is a female then I could see Zoom or Atrocitus, even Ocean Master or Black Manta would be great. If the second hero is male then I could see Star Sapphire making it in or possibly Poison Ivy.