Cool idea!

I've got a few thoughts for features, if you're open to them:

1. Ability to specify where your "played" voice resides in the voicing: As the bass note, as an inner voice, or as the top line.

2. Options for first species, second species, third, florid, etc counterpoint for each of the generated voices. Ex: You play a single note and the upper voice plays two notes for every one of yours, etc, etc.

3. If you want to get real fancy, make the generated voices perform a canon of your played notes.