I'll just preface this by saying that although I think this theory could be true, I do not believe it will ever be referenced again nor will it ever have any relevance to future content. Black Ops 3’s campaign was quite poorly received and mysteries like this usually don’t get answers anyway. This is just for fun.
This theory doesn’t have any definitive smoking guns, just a ton of small points that add up to my conclusion that Reznov is Corvus. This is all very speculative and subjective.
I originally got this idea from @gfn21 in his thread but there’s a lot more evidence that wasn’t covered there.
First, we should note the clues from BO1 that suggest that Reznov did not die at Vorkuta. The existence of John Trent in the BO1 terminals makes me confident Reznov didn't die. On top of that, we have the phonetic alphabet from the beginning of each BO1 mission which says: “Reznov is dead. Or is he dead? There was no body. Is he who he says he is?" Now onto Corvus, and why I believe they are connected.
SP/CORVUS was a CIA black project experimenting with AI and the Direct Neural Interface. They intended to use the AI to monitor people’s thoughts and prevent terrorist attacks. The torture that they inflicted on the test subjects created a hivemind out of the subjects and the AI, and in its rage it released Nova 6 gas, killing everyone present.
Importantly, we see that one of the test subjects has influence on the hivemind of Corvus. Jae Xiong was an assassin used as a test subject in the project, and we see Corvus force Taylor’s team to use Xiong’s preferred method of assassination. I believe that Reznov was doing something similar at various points throughout the campaign.
We’ll start on “Demon Within”, the mission which takes us back into WW2. In this mission, we are inside the memories of Sarah Hall, who extensively studied the Siege of Bastogne.
There are two collectibles that can be found in Bastogne that are completely out of place. A Russian Field Compass, and a Russian Ushanka Hat. The Soviet Union had zero known involvement in the Siege of Bastogne. Why are these here? These two collectibles could represent Corvus infecting Hall, and specifically Reznov’s influence in Corvus. Especially the Ushanka Hat, which became Reznov’s calling card in Black Ops 4 -you were forced to collect the Ushanka hat in Blackout in order to unlock Reznov.
The mission then takes a strange turn, as it suddenly transforms into Stalingrad. The exact area of Stalingrad that is present in the WaW mission “Vendetta”, the first time we ever see Reznov. Whose memory are we in, exactly? Why would Hall have memories of the battle of Stalingrad? This must be the memories of a Corvus test subject, and how could any of the 2060 era test subjects have vivid memories of WW2 stalingrad?
“Vendetta” also ties into a common motif of Corvus- birds. Corvus constantly shows the Player ravens as well as crows. For an interpretation of the ravens, I highly recommend this video, which is just overall a great video on BO3's campaign.
And as for the crows, I believe it’s another hint towards Reznov. In “Vendetta”, the first time we see Reznov, we also see crows picking apart the dead.
This may be a particularly strong memory for Reznov.
There’s a couple additional pieces of evidence in the data vault.
We see that SP/CORVUS considered Vorkuta Labor Camp as a candidate for test subjects.
We also see Sebastian Kreuger, head of the project, asking the CIA for records on Reznov as well as many other Black Ops characters.
The motivations of Corvus are quite similar to Reznov. Both were victims of mind control experiments, and they hypocritically chose to control the minds of others to serve their own ends- tracking down and getting revenge against their enemies. Revenge is Reznov's main objective going back to even WaW, and it's also what Corvus wants.
The last mission, “Life” has a few more clues towards Reznov. We see Kane lured by Corvus into dying the exact same way that Reznov saw Dmitri die. As this happens, Corvus says “I… was born in the minds of others.”
The pre-mission scrolling text of Taylor’s journal offers the most blatant clue towards Reznov. "Life's" entry is the only one in Taylor’s journal where he is infected by Corvus. Taylor is experiencing a jumbled mess of memories, but one string stands out:
D R A G O V I C H K R A V C H E N K O S T E I N E R T H E S E M E N M U S T D I E
There’s more to this related to blackout/spec HQ but that’s even more speculative than this has already been, so I’ll refrain for now. Hope you enjoyed. I'd recommend anyone who's a Treyarch lore fan to give Black Ops 3's campaign and Data Vault a second look, I think it's super rewarding if you're willing to look into stuff.