I would like the abillity to group tracks into userdefined sections. Bass, Drums, Lead etc...with the abillity to select a trackgroup form a combobox (drop down list) [all (default, bass, drums, lead, effects, ...] or something like that.
The idea is that ModPlug only shows the selected tracks as a group instead of all tracks. This would IMHO make managing a song a lot easier since you don't have to be bothered by all other things and can concentrate on a particular section.
If such a feature is implemented it shoud be uneccesary to say that it's a must to be able to mute individual groups or at least solo your group.
Most of the functionallity to support such a feature allready seems to be in place (channelmanager).

in addition to this it would have been nice to have a separate volum control for a group of tracks

(se my other suggestion about channel volumeboost)