I would love to have an “offset” option that determines how many octaves the upper manual (qwer…) is offset from the lower manual (zxcv…). An offset of “1” could be the default (as it is currently mapped), but 2, 3, … would raise the upper manual additional octaves (to play a higher lead or piano line on top while keeping low bass notes on bottom), “0” would be in the same octave (useful for certain mono/legato patches), and negative numbers could reverse the manuals (if a person wants to switch which hand is higher on the keyboard). Alternatively, an offset amount could represent semitones (+12 being default), but I’m not sure how difficult it might be to move the “black key” assignments like that would require.
The other thing I’d love to have is the ability to assign each manual to a different MIDI channel to easily play one synth with the lower manual keys and another synth with the upper manual keys. I appreciate how easy it currently is to use for basic MIDI recording, but I think these options would make it even more useful when on the road without external keys to play.