You probably need to clarify what you mean by content protection to get a meaningful response.
If, on the one hand, you’re talking about preventing people from grabbing assets you’ve used in your domain and doing stuff with them, then it’s pretty much a lost cause. Assets have to be downloaded to your machine and copied to the GPU in order to be displayed at all. There’s no secure path to do this, and there likely never will be. Even if we somehow managed to create a completely secure path from the network hosting to the local machine (including caching) all the way to loading vertices onto the GPU through OpenGL, Direct3D or Vulkan, there’s still nothing that would prevent someone from being able to intercept the data at the point where it gets passed to the GPU (see this Stack Exchange question for how people use software to rip 3D geometry).
On the other hand, we can work on embedding support for signing content and provide a mechanism for attribution, and this kind of work is definitely on the roadmap. The idea here is that you’d be able to push content to the (or ‘a’) marketplace and when that content is used, people seeing the content would also be able to inspect it and see where it came from and whether the use was authorized or unauthorized. For example of how this might work, you own a domain, you go to the marketplace and buy a chair model and put it in your domain, and people visiting can see the chair and if they want can examine some kind of ‘properties’ information about the chair and see it’s origin and that it was legitimately purchased. Of course, this doesn’t stop someone else from grabbing the chair model and putting it in their own domain. However, unless the model was altered in some way, anyone would be able to see the chair origin and that the use was unauthorized. Perhaps a conforming client would either flag the unauthorized content in some way, or even refuse to render it, but that’s a significantly harder problem. Even if this were the case, it would probably be trivial to side step because a person could open the model, change one vertex by some trivial amount and then save the new model, avoiding easy identification.
So if the question you’re really trying to get at is “how do prevent people from stealing and using my content?” the answer is, unfortunately, you can’t. This is one of the the fundamental truths of the internet, and it’s something people have tried to address over and over and over, with text, images, audio, and video. 3D/VR content isn’t going to be any different. So the real question is, “How do you survive in such a lawless wasteland?”, to which the answer is “Make it easier to act in a legitimate manner than an illegitimate one, and wait for enough people (and unfortunately, lawyers) to show up to make being reputable an important thing”. When consumer internet became fast enough to trade audio, people illegally traded music, until businesses wised up and made it cheap and easy to purchase and download DRM free music. Same thing with video. Yes, music and video piracy still exist, but for the majority of users, its usually easier to just stay within the law than to do otherwise.