I don't know if it has been suggested yet, but I would like to see the user being able to input a list of chords for the reharmonist to choose from, and the reharmonist can only select a chord from that list when harmonizing a melody.

To piggyback off my previous post, another similar option would be to have BIAB select substitutions based on chords you designate. For example, for the C major chord, have BIAB only substitute the Am or Em chord.

I think this would be valuable when you want to give a guitarist, who only knows how to play certain chords (and/or maybe has a difficult time playing barre chords, ie. young students), a piece of music to play that they can handle. For example, a guitarist who only knows how to play simple open string chords such as C, Dm, D, E, Em, F, G, A, Am.