Many other daws offer some functionality similar to this. Best example I can think of is Ableton Live. FL Studio supports channel groups.
Basically it’d be nice if I could collapse all my percussion tracks when I want to work on the melody or vice versa. It would be a way to help manage the “horizontal clutter” inherent with a program like renoise. Would save a lot of horizontal scrolling. As far as I imagine, this is a purely cosmetic concept, although it might be cool to have features like solo/mute group. It would be part of the pattern editor, the pattern sequencer, and mixer view.
Track DSPs already have a collapse function, why not extend this idea to tracks and add the ability to group them?
Finally registered my copy of Renoise about a month ago. For something that runs on linux, I have to say I’m never looking back. Cheers.