User blog comment:Eobard95/Prometheus suspects/@comment-3221148-20161022191814

Tommy Merlyn: Seems like a logical candidate, and getting resurrected by the Lazarus Pit would explain why he suddenly got a taste for murder. However, if Marc's comment about Prometheus "training to defeat the Arrow for five years" is not a mistake, it's impossible for him to do that, since Malcolm rezzin him would happen in early Season 4.

Roy Harper: Doesn't seem like someone who'd want to kill Oliver. Also, if Marc's comment about Prometheus "training to defeat the Arrow for five years" is not a mistake, it's impossible for Roy to be Prometheus.

Billy Malone: Malone was a minor criminal, and later a fake identity used by Batman. They could try using this theme of "Malone being a fake name for someone else" and have him turn out to be the relative of one of Hood's Season 1 victims (Starting with, say, Adam Hunt).

My own (trollish) suggestion, courtesy of one of the latest metahumans on Gotham...

Clone Oliver Queen: Remember how Moira/Malcolm hired a gang of Red Mask thugs that threatened to kill Oliver in the very first episode? Imagine if they found out what he knows about the List and had to kill him. After that, Malcolm would send a Clone Oliver for Moira to think it's her son. In secret, he prepared Clone Oliver to kill the original for years, muahahaha!

Trollin' even further...

Laurel Lance: Sara rezzed her inbetween seasons, and now she's mighty pissed at the Green Arrow for letting her down and training some new losers to replace her. Being Laurel, she's incredibly protective of her connection to Oliver, which is why she targets Tobias Church and threatens him not to touch Oliburr.

Yyyyyyyyep.