A nice feature that I would love to see is the ability to independently move the direction of the lighting. This is a really useful feature of software such as Meshlab and CloudCompare for cultural heritage applications.
For example, with a 3D scan of a gravestone it can be easier to read the inscription if the lighting is moved to a low angle. Currently on Sketchfab (unless there's something I've missed) the only way to do this is to rotate the model. This makes reading writing or seeing decorative detail difficult.
On the Meshlab iOS app, there's a button to tap which enables a lighting mode where swiping across the model changes the lighting direction rather than manipulating the geometry. Could a similar feature be enabled on Sketchfab?