I like .ply, it's an easy format that supports vertex colors! .ply files are often used for point clouds, so it could be good to consider point cloud rendering when importing .ply files as well.

Might be able to find a nice library for this on here:
https://github.com/nothings/single_file_libs

There should be a method that can calculate the size of the panel under an outside model and the initial position of the user when entered the view of any model from the menu.

Log files do not contain Cluster score type information:

Ensemble Results
----------------

Cluster method: spicker
Cluster score type: None       <-- HERE
Number of clusters: 3
Truncation method: percent
Percent truncation: 5
Side-chain treatments: ['polyAla', 'reliable', 'allatom']

Gemmi has significantly more documentation and allows you to do things like modify PDB files and read all sorts of reflection files.