I gave some thought to what needs to be improved in partial regen and here is an issue, that I believe is the most significant.

When selecting a section to be regenerated, BIAB is not respecting what happens before or after the phrase. It would offer pieces to match chord, but often not natural to a larger segment that your selection is a part of.

I believe to tackle this, there should be an adjustment of selection - "interpolation". Meaning BIAB would look ahead and beyond regenerated selection with X (user adjustable) value and come up with a part that is more natural for part selection being regenerated. It seems it's doing that for larger selections, but with granular, sometimes it takes a lot of wrestling to get it right, including unwanted repeats. Also, there should be a second adjustable value to determine of how much of end points BIAB could change for natural transition.

The difference between 2 adjustments I propose is:
1 will just look "ahead"- non destructive.
2 will determine of how much can can be changed beyond either side of selection for natural transition.

I hope that this makes sense.