I have an observation in this regard.
The current system of "genres" is inadequate, not merely because it is extremely limited in choices. It combines true genres (types of pieces) with categories based on performing forces, e.g., "opera" is a musical genre, but "piece for string section" is a performance category.
To illustrate: "piano piece" is not a genre -- the term simply says that the piece is for piano (although, interestingly enough, it doesn't have to mean "solo.") "Piano pieces" include many genres: preludes, fugues, scherzos, character pieces, sonatas, etudes, etc., and these genres can be found in music for other performing forces.
Admittedly some genre terms do coincide with performance terms, as with string quartet.
Just some thoughts.
"A libretto, a libretto, my kingdom for a libretto!" -- Cesar Cui (letter to Stasov, Feb. 20, 1877)