I agree with Brennan, that this is not an easy to structure feature, having consequences to the whole design.
So indeed firstly we would need to have some settings per part, see also my post about a song matrix. Two flags would do for this purpose: Once-Off. When nothing is checked nothing extra happens.
- When Off is checked this part is been disabled (muted, not editable, skipped).
- When Once is checked: in playback/overdub/record when reaching the end of this part, then one returns to the beginning of the previous part (the one we came from, if not defined then the first part). When unchecked the part loops, like it is now.
For me this feature has not a high priority, however it is a good idea.