Thanks! Yeah, I've been asked about this a few times - however, it does look like this is basically exactly that (using ARKit and ARCore) and exists already: https://denchisoft.com Have you heard of this tool?
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VtubeStudio plus an iPhone for mocap is the standard for the big-time vtubers that use 2D avatars, however 2D avatars actually have quite a high barrier to entry because pretty much your only option is to have something commissioned. 3D avatars tend to be what nearly everyone starts out with because there are several free programs out there that can help you make a decent starter avatar.
VtubeStudio only supports the nicer 2D avatars and as far as I know has no intention of getting into the 3D side of things. There are a few decent programs people use with 3D avatars(links below), but it seems they aren't really as high quality as VtubeStudio so they don't have the market cornered like VtubeStudio does for 2D.
As far as tracking goes, on the camera side of things there isn't really any difference between 2D and 3D that might limit you to one or the other.
There is a pretty large demand for tracking apps on Android because there are no widely used apps currently available. Big-time vtubers usually get iPhones so they can use the ARkit tracking that is Apple only, but a lot of people just starting out have Android and are currently forced to use a regular PC webcam that tends not to be as accurate and also doesn't allow people to offload the computing resources needed for face tracking to their phones.
Vroid Studio(the main app people use with 3D avatars): https://store.steampowered.com/app/1486350/VRoid_Studio_v201...
Warudo(this one is a lot newer than Vroid Studio so it has a lot less users, but is probably the second most popular 3D app): https://store.steampowered.com/app/2079120/Warudo/