There are 2 reasons why I’m against the idea of creating yet another scene file format. First VTK does not have a scene graph so a “scene” is not a core part of VTK : creating a new VTK specific file format for something that is not core sounds bad to me.
Second there are aready plenty of very good scene file format. I’d prefer to improve existing support for common scene file format such as glTF or USD. If we see that we need to store some custom variables then we could always design an extension for these file formats and support it in the importers / exporters.
But this is a bit off topic though 
if needed then we can create a new topic