Sounds good to me. I’ve always thought that one of the misleading and confusing elements of Rhino’s block management is the ambiguity between definitions and instances. I guess a user can get used to it eventually, but it sure complicates the journey to familiarity and just doesn’t seem neat and clean.
Your proposal would certainly help a lot in keeping users aware of when they are working on a definition instead of an instance. I think there would be a great benefit to having a standard unified approach to working on a definition and a definite differentiation between definitions and instances.
I think your proposal also implies that when the definition editor is used it would also only be able to open definitions, which would bring an organizational benefit by listing definitions in an exclusive list.
Another useful feature would be the ability to list/highlite in view the instances of a selected definition.