Mid-note velocity changing ("automation")
Currently (at least in 2.3) crescendos and diminuendos only affect the velocity (volume) at the start of the note. That means that if you wanted, say, a long violin note to slowly fade out, and placed a diminuendo along the note (putting dynamic signs at the beginning and end as per usual), it would just play the note as if the sign never existed. If this feature were added, it could drastically increase the playback quality of all future pieces.
The way I propose implementing it is as a property of the crescendo/diminuendo. For example, you wouldn't want the piano to fade out unnaturally fast since it's a percussion instrument, but a dramatic brass accent could use it. So the feature is off by default, and would need to be turned on for every individual de/crescendo symbol.
I understand that it could be a very difficult feature to implement (probably very widely requested as well), perhaps requiring a rewrite of the code or something. But, my hope is that in the future, perhaps when everything that can be done on sheetwriting is done, Musescore will begin to focus on its playback aspects in order to become a unique music-making tool.