I'd like to start off by saying: wow. This campaign was very interesting, and the entirety of the Frozen Forest scenes blew my mind.
However... I pieced something together throughout the game. It may sound very odd, but I have a feeling that Corvus is Viktor Reznov. Hear me out here, because there are a lot of little details to make up this analysis.
Let's start with the idea that Reznov was the beginning piece of the Corvus program. This would mean, rather than being killed in Vorkuta, he was captured and underwent the process that made him into what he is. Reznov was able to manipulate Mason through mind control, just as Corvus does to Taylor's Black Ops team.
Now for the fun part: the evidence! First, look at the mirrors of both the characters in the game and the major events. Right from the beginning, you miss extraction and are left behind defending your escaping squad. This is very similar to the situation with Alex Mason in Black Ops. The player can be compared to Woods in the sense that they followed the orders given to them, while Hendricks resembles Mason in the fact that he gives in to his subconscious. Kane was similar to Hudson's CIA role. The biggest evidence in the "mirrors" portion is the player's character watching their close friend Kane die from Nova 6, exactly how Reznov watched Dimitri. Corvus specifically tricks Kane into dying this way, which is crazy. For some reason, Viktor is recreating this painful moment from his past (Notice the brief Black and White here, too). Why would Nova 6 even be in this area to begin with?
Now, for some additional clues. The World War 2 battle during mission 8. Why? I understand that Hale studied and wrote about that battle, but it seems like such a clear hint to Reznov remembering his past as well (The area outside of where you fight the zombies in this mission looks very similar to the sniper mission "Vendetta" from World at War, another possible connection. That could just be me seeing things, though). There's an odd use of crows in the final sequences. The only connection with other Treyarch games that comes to mind is, again, the "Vendetta" mission where the crows are eating bodies in the opening sequence. This was another major memory for Reznov Finally, this line of dialogue from Hendricks when you enter the Frozen Forest for the last time. "...let's call it Corvus, after it's place of birth..." A quick Google search of Corvus, Russia leads to... you guessed it, St. Petersburg, Reznov's birthplace.
The final minutes in The Frozen Forest are ENTIRELY focused on the concept of who Corvus is. It doesn't accept the fact that it's just an AI, and apparently neither does Taylor. He begins yelling "Who are you!?" as well. It seems to cut off right before a big reveal. Take the disguised voice away from Corvus's dialogue in the end, and picture Reznov saying those lines. The smoky presence is gone, and we see the real reason for the corruption of the hive mind. Resnov. Somehow, he found away to sabotage a future Black Ops project like he did during the code war.
Could there be a clearer ending? Perhaps after beating the story on Realistic, which awards you with a trophy/achievement called "Nobody Will Believe You"? Seems like an odd title and appropriate name for this theory... Perhaps somewhere in the Data Vault?
Update: recently went through Taylor's Journals again. The final entry for the "last" mission, Life, is a mess filled with jumbled dialogue from the campaign as well as code deciphered using the phonetic alpabet. By now, members of the community have deciphered it, and one message DEFINITELY stands out:
Translation: DRAGOVICH KRAVCHENKO STEINER THESE MEN MUST DIE.
This last entry does not seem to be Taylor's alone. Rather, it appears to be Corvus breaking down. This is huge. Why would Corvus even know of this saying? It simply does not fit, unless Reznov is a part of Corvus, just like the other victims. I do not know how they would have the technology back in his time, but it's the best explanation I have. It does not make sense for Treyarch to throw in a random easter egg here, when these journal entries have helped to reveal the true story to us so far.
I'm sure there is tons of evidence for this that I passed up, and even more that helps disprove it, but I thought it was worth sharing. Thanks for reading!