Would it be possible to add parallax to tree billboards? I feel like that would look very good in SkyrimVR without sacrificing much performance.
Another thing that might look good is multiple textures per billboard, for instance, 8 textures for each tree. Each one would be for a different angle and they would swap textures based on which direction you are looking at it from. This would make trees look 3D, without having to render the entire thing.