I am a fan of Paraview and a student of GLVisualize.
In my usecase, I am trying to make and visualize mesh node data, including scalar and vector fields defined over those nodes. Paraview does an excellent job at that using extremely few commands, in the Python API. In that area, I think GLVisualize still has some way to go. I also find Paraview faster. So I would like to suggest to the developers of GLVisualize to see what they can reuse from the Paraview C++ libraries, to bring the best of the two worlds.
Some formats I think would be useful to freely read from and write to in order of priority are: vtk (all variants), pvd, stl, step, obj, and fbx; reading into arrays of mesh element and node structs with their scalar and vector field properties, where applicable. WriteVTK does a great job at writing vtk files, but I don’t know if any similar package exists for reading vtk into arrays of raw data or structs.