What I would really love to see is the ability to piece together a 'custom drum set'. Ideally, I'd be able to piece together a staff of pitched percussion instruments tailored to the key of the song.

One example of this would be:
Pitched Ride Cymbal 'A'
Pitched Clap 'A'
Pitched Clap 'F#'
Pitched Clap 'D'
Pitched Crash Cymbal 'D'
Pitched Snare Drum 'A'
Pitched Tom 'A'
Pitched Tom 'G'
Pitched Bass Drum 'D'

There could be some predefined instances available ("latin percussion section", "marching quads", etc.), but it would certainly be useful to have the flexibility to define your own.

I would _really_ like to see support for pitched percussion, and I think that this would be the best way to do it. Since a piece would generally not call for every pitch from any given percussion instrument (most often only one), being able to mix and match several different instruments onto one score would be quite useful.