I would like to suggest some features which can make Sketchfab more closer to game engine.
It will be very usefull when Sketchfab team will implement this.
1) Atmospheric: Fog can be non-uniform and layered, with ability to change global color and change colors depending on distance (Gradient map based).
+ Ability to put the fog in containers (gizmos) of any shape (containers - it is a simply polygon mesh with non renderable properties)
2) Physics of static meshes - camera can not move through the walls. It is very suitable for closed game levels such as corridors, bunkers etc
3) Baked animation from 3D editor
4) Triggers which can open doors in the level or turn on backed animation of some mechanisms etc