here are my first two tests at animation to sketchfab. The first was a quick easy test, the second one I wanted to try something a bit more complicated.
Opinions after testing: I wish I could hard set the speed of the animation so when people look at the model like my second one I posted its not spinning to fast. It should really be set at 0.1 when playing it back. I know I could easily fix it and reexport it but with lots of material setup thats just a pain. So hard set animation speeds in settings would be awesome.
Another thing that could be cool is being able to just export animations in and place them to the model but also being able to play multiple animations at once this is how I setup a animation blueprint in Unreal Engine and blend them to control individual animations. This may be to complicated of a feature for sketchfab, I havent heard how serious you guys are about it or if its just extra fluff for single animation tracks. Allowing people to take my first model and switch between animations could be cool he has over 20. On the second model blending animations would be cool because then I could spin different rock clusters at different speeds. I know its still beta, but just my two cents. I will say the 3d max to sketchfab was seemless after I figured out the edit mesh trick and the double export trick cause I love to use helpers as bones:)