Here are a few thoughts I had recently while using Sketchfab.
Animation Timeline Interface
The way it works now is very similar to what we are use to with web players such as YouTube where when you hover of the picture, the timeline pops up. But because the media is more interactive here on Sketchfab and the cursor is generally always on the picture for navigation, the timeline seems a little more in the way when watching an animation and looking the model over.
My suggestion is to have an invisible area at the bottom of the screen that will bring the timeline up when hovered over. This way the majority of the picture is open to scroll around with and if we wanted to interact with the timeline, it would be a matter of bringing it up by moving the mouse down towards it. This is so I can play the animation, look all around, and still have 100% of the screen dedicated to the model until I want to interact with the timeline.
The smooth camera movement in the Sketchfab viewer feels best when swinging the camera around quickly and release, centering on the model when the scene first loads, and for the transitions from angle to angle by viewing annotations or double clicking a surface. Where it starts to feel a bit sluggish is while moving around normally. Compared to scrolling around in other 3D view-ports where the movement is more or less one-to-one, viewing models on Sketchfab feels much less crisp than I think it should.
It could be a matter of preference but that's how I feel about it and I would like to know your thoughts.
Those are all the suggestions I have for now!