My first thought was Abaddon as well, but as much as I’d like to see that happen I doubt they’ll bring him back. Assuming it isn’t just Token-ominous-narrator-voice, and is actually a character, I think it could be (read: I hope it will be) Menzies.
The guy fits the bill as potential “power that was manipulating Scarlet all along”, and I’m all for bringing more of the gw1 lore into gw2.