Rubato: ritardando and accelerando
Lots of musical styles use speeding up (accelerando) and slowing down (rallendado or ritenuto). It would be great if a musescore could, from a selection of beats, say "slow down from this tempo to that tempo" or "speed up from this tempo to that tempo" or even set a percentage.
Right now Anvil Studio does allow setting tempo (not accell, or rall.) and Finale does support all these, but its hard to do.