To make MuseScore really, really useful to teaching Choral harmonizing in gymnasium in Denmark (where I teach) support of function-harmonic notation (don't know the correct English term) would be a killer feature rendering all other notation software obsolete.
The ability to choose stuff like
D (for dominant)
D/ (slashed over the D)
64D (where 64 is above each other in small chars before the D)
Sp (for subdominant-parallel, where the p is small and below the S)
would be amazing. I could provide a complete list of these functions, but you probably know them already.
As you don't have a "bounty"-feature here I can promise a 100€ donation for this feature. Actually a page with wishes and prices would not be a bat idea, I'm sure I could get my school (or myself) to increase donations considerably for specifik features.