Humanize MIDI
The "Humanize" feature on MIDI recording programs intelligently (depending on the program) staggers the notes in time to make it sound like a human instead of a player piano. It's adjustable.
You could export MIDI from Musescore and then humanize it in another program, but it would be swell if Musescore could humanize it both while playing, and when exporting.
I haven't used Musescore yet, but the synth documentation shows a really finely tuneable instrument.
I'm sure that if the team took up humanization, they'd make a really fine module.