Ability to use 16va and 16vb instead of the default (and weird) 15.
In Musescore, octaves and double octaves are signified using just a "15", with an up or down bracket.
Philosophical differences notwithstanding, using 16va and 16vb more accurately reflects what is in certain scores. A score written in the 1920s was not written by a programmer. 8va, 8vb, 16va, 16vb, these are things I learned in school and which adorn countless music textbooks. The programmers and technologists may not like that, but there it is. If Musescore professes to be the premiere engraving software, it should provide this functionality. Otherwise, simply have a disclaimer splash screen that says "Our way or the highway."