First of all, I love PLAYBOX. It's a great tool for ideas and cool sounds, but the work flow could use an upgrade or two.
1) Mute/Solo the 6 sources individually.
2) Export of the stems Drag n Drop.
3) Function to use more keys to e.g. play your own chords (from C1-B2 or something). Keyboard Mode vs. Chord Mode.
4) Playbox FX as a separate plugin. I would pay money for this! I think the FX section is insanely useful, but I would love to use it on other instruments as well.
Just my 2 cents...