Somehow such a feature should tie in with mix recall i.e. you should be able to select regions in a project and batch export it with a selected mix e.g. scratch tracking of several songs in one project. song A needs more vox, song B needs lower vox, thus mix quickly and save mix scenes, then export region #1 with mix A and region #2 with mix B ...
maybe a simple way to achieve this would be to have mix recall (optionally!) remember the timeline and tracks selection???
I would use this lots, too, for bouncing rough mixes plus mix stems for rehearsal play-alongs from scratch arrangement.