Here is a list of characters who I think Prometheus may be
This is who I'm hoping Prometheus is. The theme of this season is legacy and Oliver's mission to maintain his own, as well as honoring Laurel's. What better villain to use than the literal image of perhapes Oliver's greatest failure, Tommy. Tommy died as a result of Malcolm's undertaking true, but the point of Oliver's crusade was to honor his father's wishes which meant stopping the undertaking and he actually failed and Tommy died as a result. Maybe Malcolm or perhapes someone else who wants Oliver dead revived Tommy somehow, and brainwashed him into becoming a Reverse-Arrow type vigilante. It has also been 2 weeks into the season and we haven't heard anything from Malcolm, which I find very suspicous. Maybe one of Tommy's drives is because he died saving Laurel and 3 years later Laurel was killed by Damien Darhk, when Oliver had chances to kill him before and failed, or something. I'm honestly not sure how it would work but I think it would be awesome, and sad, if Tommy was Prometheus as Oliver would have to fight his best friend.
Roy Harper / Arsenal
Arrow copies Batman all the time, this is no joke. Season 3 was basically 95% of the Batman bible but I've gotten used to it. Maybe one of these "adaptations" is the Jason Todd / Arkham Knight story from Batman: Arkham Knight. To me Prometheus actually does seem to be an Arkham Knight like character, and if that's the case then who better than Oliver's first major apprentice. I should also note that Oliver is now training more team members, and one of Jason's motives for revenge as Arkham Knight was his anger towards Batman for "replacing" him with Tim Drake as Robin. Now Oliver has been training more recruits and just as he decides to do this, Prometheus shows up. We did see Roy last year for one episode where, aside from behind extorted by Noah Kuttler to aid him, he seemed ok, but then again the timeline has now been altered by Eobard Thawne. Maybe one of these changes is that last year's episode with Roy didn't happen and the last time we saw him was when he faked his team in late Season 3. This would make it roughly 2 years since that happened, enough time for something to happen to Roy enough to make him want to kill Oliver for whatever reason. A stretch I know but we do have magic, meta-humans, Multiverse and time travel in this universe of shows so, anything is possible.
Detective Billy Malone
I don't trust this guy. It's not because he's with Felicity which I'm sure is what were supposed to feel, but considering I hated Olicity and I'm glad it's finally over. What makes me suspicous of this guy is that he shows up a few months after Damien Darhk's defeat when Green Arrow became active (as Green Arrow as to the public he and "The Arrow" were different people) just as Prometheus shows up. Also he's a detective and Prometheus first act was murdering a cop, specifically one of the non-corrupt cops in Oliver's team. Maybe I'm wrong but then again I didn't suspect "Jay Garrick" (Hunter Zolomon) was Zoom, and I was wrong. I just don't trust the new guys as I'm fairly convinced Julian Albert either has a connection or is Dr. Alchemy. Either way I hope I'm wrong because of Malone is Prometheus it would kinda be a bit predictable of every new guy is the main villain of the series.