In normal situation, one instrument is associated with 1..N tracks. A toggle that tries to maintain this association as well as possible would be nice. With the toggle off, the behaviour would be exactly like it is now. But with the toggle on, sointu would:

1. Delete all tracks associated with the instrument while deleting an instrument.
2. Add voices to track/instrument when adding voices to instrument/track
3. Add new track when adding a new instrument
4. ....

If the association cannot be maintained, sointu would just give a warning and the user is left to sort it out the track <-> instrument association in the old way. In particular, problems could arise when reordering tracks or instruments.