Basically what you need is to upload a mesh to your server en whenever a player needs to see it, download that mesh?
Idk how to help with this specific, but some things to keep in mind:
- If by devices you mean mobile devices it may get tricky since it will consume a lot of space on the phone.
- It probably won’t happen during runtime as downloading a model + textures takes time (do like GMod and download it during loadingscreens)
- There is a reason why MMORPG only do a custom profile photo and not a custom mesh with texture. Most of the time the photo needs to be a certain size too (steam profile pic, Tera guild pic, etc…).
if you still want to try it, I think GMod comes closest to what you want. Try to see how they did it