I'm sure there is much that can be improved with the blend file thumbnails, but this does only one small thing: remove the "fake passepartout."

Right now the thumbnail is created and then the top and bottom portions are partially obscured and an (ugly) dotted outline is added, in an attempt to mimic Camera View. This is done even if you don't have a camera in the scene, this is just added as a visual flourish and decoration.

I think most users would prefer to just see more of their scene. The following illustrates the change, with top being current code and the bottom showing how they look after the patch is applied:

Note this change is also a part of https://developer.blender.org/D9404, as I am breaking that one into pieces.