I've been fascinated with the flexibility of the Ondes Martenot, but no one has been able to create a Midi controller that reproduces the abilities of this keyboard. The Ondes Martenot has 2 unique features:

1. A finger ring with a pulley that allows you to bend a single note across 4 full octaves on the keyboard. I realize that this is probably impossible with current MIDI technology (the best you can do is use note bending across 2 octaves), but this isn't the part that interests me. It's the feature below.

2. A keyboard mounted on springs. If you vibrate your finger (like the way a guitarist does on a fretboard) it will bend the note. Lean right...it bends up; lean left it bends down. It seems like this would be relatively easy to build and would add a whole new form of expression from the keyboard.

So, my suggestion, build a MIDI controller that supports Finger Vibrato. Maybe add a break so you can lock it in the off position if you don't want it. Otherwise, it will make playing sounds like violin SIGNIFICANTLY more expressive, and while the Ondes Martinot is monophonic, there's no reason why a modern keyboard shouldn't support polyphonic finger vibrato.