Triple sharps and triple flats
While triple sharps and triple flats may be somewhat, if not, very rare, they are nonetheless existent. From time to time I experiment with Musescore, and even before I discovered this software, I composed a short piano work in G-sharp major, which requires an F-double-sharp in its key signature. Triple sharps and flats are a must have for Musescore for obvious reasons like one I'm about to explain below.
In the above paragraph, I mentioned my work in G-sharp major, which would be the enharmonic equivalent of A-flat major, only with sharps instead of flats. With that in mind, I thought it would be a good idea to make it so that, based on the whims and moods of individual composers, that all keys with flats have enharmonic equivalents with sharps, and vice versa. C major and A minor, being the only two standard keys with an all-natural key signature, have the distinction (in my mind at least) of getting both a sharped enharmonic equivalent (B-sharp major, G-double-sharp minor), and a flatted one (D-double-flat major, B-double-flat minor).
For G-double-sharp minor to work like any other minor key in common musical conventions, an F-triple-sharp would be required for effect, especially in harmonic and melodic minor scales, chords, and in standard musical progression. Similarly, one must typically use a triple flat if he wants to create a G-double-flat minor chord in a D-double-flat major context.
I know there must be other uses for triple sharps and flats, but no further examples come to my knowledge. I know they're out there, and although it's impossible to know everything, I'd say they're a must have for Musescore.