Wish: RenderContent type for arbitrary datatypes

It is great that we are able to use RenderContent-derived classes to represent our types, and have them managed by the document, allow them to be linked together, and so forth. However it seems an oversight that they must be conceptually of type material, texture, or environment, when many things we need to represent have nothing to do with those categories. I would request addition of a 4th RenderContent type for representing arbitrary datatypes, with a corresponding panel for creating/managing/editing them, as with the existing 3 types.