This is the one I think is most likely.
There's a VHS called Report: Void, which details a doctor (Dr. White, I think, he seems to be the nicer of the two doctors we're aware of based on his line delivery. The name was mentioned in Samuel's last day so I'm just going to use it here for ease.) is reporting on subject 1322, aka Kevin. Based on the number he's one of the more advanced experiments, and I assume Void is the name of what he eventually became. In this tape he's still human, but for his monster/toy form, we can look to another of the VHS tapes.
In the Claire Harper Debrief vid, she mentions Marie waking from a Red Smoke-induced nightmare and saying she sees something "colorless". Claire herself cannot see it. Now, this could be the nightmare/skeletal CatNap, though I think Claire would have noted Marie looking at CatNap frightfully as she spoke if that were the case. I've heard some people guess it was the Prototype, as "colorless" might fit him, but I also think this is a stretch. BUT! Colorless would fit something called "Void", no?
I think Void is something that can only be seen when under the influence of the Red Smoke. (Given that his higher experiment number, he may have abilities that feel even more supernatural than the monsters we've already encountered.) This could be a way to add some depth to the gas mask mechanic, as in Ch 3 you really don't have much incentive to take it off other than it blocking your vision slightly. It's more like you see gas, put it on, go through gas, then take it off. Maybe in Chapter 4 we'll have to balance how much Red Smoke we inhale at any given time and make decisions on when it's "worth it" to put the mask on or take it off.
The evidence for this is flimsy. There's some kind of child-like giggle during a certain point in the Home Sweet Home section, that as far as I know is never explained and doesn't really match any characters we already know.
So what else is in the Home Sweet Home? Cardboard cutouts of Mommy, Daddy, and Baby Longlegs. They seem kinda out of place, because as far as I'm aware they were part of the Game Station, not Home Sweet Home or Playcare. Mommy and Daddy play only static when their buttons are pushed, which I take to be the devs confirming they are both dead. Baby has no button, meaning their status is unknown.
This could also be the one responsible for the giggle in Home Sweet Home. The existence of this toy may be a bit of a stretch, but Ollie has a line where he says there are "eight...I think?" Smiling Critters. It's possible this is just in reference to CatNap being removed from the lineup, or the fact that all the Critters but DogDay and CatNap are deceased. (Maybe Ollie doesn't realize that for whatever reason?) But I think it could also hint that there were originally even more planned and Ollie's unaware of who made the cut.
Now...do I think any of these will be the MAIN villain? No, not really. I feel like for meta reasons they won't hint much at the main villain until we get the Ch 3.5 round of ARGs and teasers. But they could be a side villain, on the level of Miss Delight, or a minion like Bunzo and PJ, or possibly a potential semi-ally, like Kissy or DogDay. But! I could be wrong about that too. I do think part of why Chapter 3 took so long is them wanting to get things really set up and planned out (both from a story-telling and game-design/programming perspective) for Ch 4 and 5 (because I do think there will be only 5 chapters ultimately. I feel like after that we'll get PPT 2 or some other arc). So it's possible they feel more comfortable hinting at the main villain of the next chapter this time around.
