Bell Trees/Wind Chimes
In a lot of modern orchestral music, bell trees (an unpitched percussion aka wind chimes) are heavily used. However, Musescore doesn't have this available. One factor that might make this a difficult instrument to program is the fact that they are always played as glissandi, which Musescore doesn't support either. If there is any way to make this instrument available and sound good, that would be excellent.
My initial thought would be to use the triangle sound from the soundfont and have it repeat itself rapidly while gradually raising or lowering in pitch, if that's possible.