Play only one instrument (or several selected instruments)
A cool feature - assuming it's not already present - would be to allow the playing by the synthesizer of only certain selected instruments. For example, you could play only a clarinet's part. Or you could play only the trombone section. One way would be to select the instruments you want to hear by highlighting them and then hitting Enter (rather than spacebar). The current method of selecting parts is to click on one measure, then click on a measure directly above or below so that all the parts between those two clicks are highlighted.
If you wanted to add the ability to select parts that are not contiguous - for example the bass and lead trumpet parts - it would be necessary to add the ability to use the Ctrl key instead of the Shift key to add non-adjacent parts. In other words, Click on a measure in the bass part, hold down the Ctrl key, then click on a measure in the lead trumpet's part, so that both are selected (but not the instruments in between), then press Ctrl-Spacebar (or whatever) to play only those instruments' parts, starting with the highlighted measure.