Support for large time signatures
This has only rarely been asked about in the forums (see https://musescore.org/it/node/121246 for an example), but tall time signatures are quite commonly used in scores—for film scores especially, or in any piece where there are frequent time signature changes. In Finale, they look like this:
In Sibelius, they are by default simply scaled up versions of the normal time signatures, but can be set to use other fonts including Finale's.
I have a feeling that this feature would open MuseScore up for a massive new group of users.