Your new file format scratches an itch that many VTK users have - the lack of a simple scene file format for VTK. There are a number of exporters for other scene file formats (glTF, x3d, vrml, renderman, etc.), there is even a vtkJSONSceneExporter that could be quite close to what you have defined. However, there are only a few importers (glTF, 3ds, vrml) which are too complex and/or quite old.
I’m wondering where adding this new file format will lead us. It would be so tempting to add some more features (definition of camera parameters, lights, SSAO, support for multiple objects, object position/orientation, etc.) because it would be so easy and would result in huge improvements in capabilities and rendering quality. Should we resist and keep only the current capabilities? Or should we give in and finally have a simple VTK scene file format?