Looking at the Wikipedia article on clefs ...MuseScore is missing a few.
Here's a picture of sub-octave treble clefs. The one with the 8 under it works fine, but support for the others (without manual positioning for every darn system) would be nice as well. Here's two kinds of neutral clefs, one of which MuseScore seems to be missing.
Then there's some interesting looking ones under the "History" section of that article. Weird looking F clefs. Not in any way necessary but worth a thought.