Without a second thought, it's definitely Swamp Thing for me. I'm not up on the current state of the character, which is regrettable, but I've always been a fan. Since the old movie from the early 80s in fact. I have a few old issues of Swamp Thing around here somewhere. Doubt they are worth the cover price at this point, but that hardly matters. What matters is that Swamp Thing would be such a great addition to the roster.
We all know Manhunter is going to make his way in the game. No point wasting a vote even though he is the most obvious next choice.
Move Scorpion over to the villain side so that it can have one of its two gadget characters.
Power characters (Hero):
Supergirl (as another person said, if your gonna waste a space on Batgirl, then return the favor for Superman fans. Besides, they managed to make Zod play completely different from Supes despite having the same powers as him)
My logical choice for a second power character would be Starfire.
Gadget Character (hero):
Gadget character (villain):
Mirror Master (Flash seriously needs one of his baddies playable)