My work is in computer graphics but we have recently become connected to some local museums that have begun to use light field techniques for preservation of their art and historical artifacts. They have used the tools available from the Cultural Heritage Institute (CHI), building polynomial texture maps of many of their flat works. This is a long shot in terms of 'features' for SketchFab but at present there is no obvious way to share these in a browser as you do with 3d models. I know from my experience in graphics that the engine you have already built would be well suited for this and I think there would be a big market if you could provide a viewer for this sort of data.
Even further out there, our research group has recognized that the data captured for photogrammetry (thinking specifically of Agisoft PhotoScan) is identical to the type of data used to render an unstructured lumigraph. PhotoScan (and similar software) analyzes the input photos to back out the intrinsic and extrinsic properties of the camera and all the individual views and then uses multi-view stereo techniques to reconstruct a polygonal mesh and a detailed albedo texture. For a lumigraph, you keep the polygonal mesh but instead of a single texture you retain all the input photographs and re-project them back onto the mesh intelligently (blending together the pixels from the images that are as close as possible to the view you are presently trying to render). It is a long-term project of ours to make this happen in a browser through some means of either cloud rendering or via a representation that is somewhere between a textured polygonal mesh and a full lumigraph. A lumigraph is too GPU intensive for the average consumer to render on their device.
Towards the lumigraph technique, it is very important that the original photographs are still available in some fashion and this is something that would be a more immediate suggestion that SketchFab could consider providing. Allow important complimentary data to be stored along with the more traditional polygonal model (like a spreadsheet or XML file of camera positions and the original photographs). Decisions to accommodate this meta data now would help prepare you for what I think will be the future in cultural heritage scanning!
Just my 10 cents! Big ideas but this is a BIG area with lots of exciting things happening right now. Get on this train! It's going places.