One note on any plans you make, not to dampen any excitement, but we have to work within the confines of what the MC database functionality offers, and the main concern with reading the suggestions is that the database is not a relational database, its basically just a list of files with properties. As such, we don't store any abstract "entities" like an Album, a Series, or a future Composition. They are all virtual concepts created on the fly, based on the files it's currently looking at.
This means that for example a Composition, or an Album, are not "things" in the database, as such they can't have metadata attached directly. Any information about a Composition, an Album, or any such metadata-based grouping concept, would need to be attached to the files within. For the same reason Albums don't have ratings, for example. You could make an argument for a relational Composition field that's their rating, or just form an average rating based on the files ratings, but either are of course more clunky solutions.
We're happy to work within the confines of how the MC database works to support Compositions, similar to eg. Albums, but re-designing the internal database structure to support such abstract entities is out of scope currently, I'm afraid.