Have we seen both Atoms yet? Could it be one of them?
Also, since we're not sure which stories from before are canon - because DC fucking hates its fans - could it be someone who had been dead before? Ted Kord, Ralph Dibney (trolltastic decision that would be), Aqualad/Tempest. Other Justice League members I don't think we've seen yet include Metamorpho (probably not big enough), Plastic Man (he's only been seen on a TV screen so far), Geo-Force, Max Lord ... man, everything seems to point toward Wally.
Shit.