Some reaction videos have theorized that Prometheus is Tommy Merlyn, back from the dead. But, now that they've announced Talia's presence this season, I was thinking that maybe she was Prometheus, looking for revenge for Oliver's having killed her father and turned the League over to Malcolm Merlyn. However, she'll be played by Lexa Doig, and, as petite as she is, and as beefy as Church is, I don't know that she'd be able to slam him to the ground like Prometheus did at the end of "The Recruits".
Also, Lexa Doig as Talia! Blastr's article mentioned Continuum and Stargate: SG-1 as projects of hers, but they left out her five seasons on Andromeda. For shame.