2. Support for any emerging formats that improve on .ply (https://github.com/nianticlabs/spz looks promising)
3. Better navigation controls (or at least document what you already support)
More formats are definitely on the roadmap. At first I'd like to add full support for higher order of spherical harmonics and then I wanted to take a look at other methods, too. Maybe 2DGS and/or SMERF would make sense for comparison purposes.
Explanation of controls makes sense, too. I'll add that.
Regarding WebXR, that's currently not on my roadmap. However, if the demand is there I'd consider adding it