Ok guys this isn't a full theory of Prometheus's identity it's more just a narrowing down.
Prometheus as we know is an entirely new character created specifically for this season of Arrow so that most likely means he is an amalgamation of several DC characters.
Prometheus's suit very much resembles a black version of Oliver's original Arrow outfit and very well may be because it has been established that he has access to SCPD lockup, so I think that the gear Prometheus is using (Except the mask) is ALL from Oliver's old equiptment. The sword and black colour scheme is also a callback to Malcolm Merlyn in Season 1.
Prometheus, in Greek Mythology, had a reputation for being a clever trickster who was punished by Zeus for giving the human race the skills behind Fire and Metalwork. The work trickster can also be a synonym for Magician and that is another calling card to Malcolm Merlyn and the Merlyn family. In Greek Mythology, Prometheus committed a good deed and was punished by Zeus for it. This is clearly a connotation about Oliver punishing Prometheus for something he believed was a good deed or murdering Justin Claybourne for creating a cure to Tuberculosis. I think that is what Prometheus meant when he said "This is about so much more than revenge".
Whether or not Prometheus is Justin Claybournes son, i have a suspicion that Prometheus is involved in the AK Desmond Group. The AK Desmond Group is the Black Market Organisation that weaponised Tuberculosis. The Name AK Desmond is a reference to Albert Desmond aka Dr Alchemy, but since he is appearing on Flash and most likely has no connection to Star City, I believe that it is a reference to another Dr Alchemy called Mister Element. In the DC Comics Mister Element is an Assassin who slightly resembles Prometheus and is a member of the Keystone City Police Department who secretly murdered cops. I think Prometheus is an amalgamation of Mister Element due to the above facts. Mister Elements real name is Alexander Petrov, the name Petrov has Russian ancestry and is a hint towards Prometheus being trained by Talia al Ghul who trained Oliver in Russia. Also the creator of Mister Element is Geoff Johns who is also a writer on Arrow.
I personally believe that Prometheus is someone we have seen on Arrow already because he is still wearing his mask, so that way we will have a dramatic unmasking moment in the future.
Let me know what you all think in the comments below