It'd be interesting to see a fantasy layout: say you were to let go of the computer keyboard layout, how would you set it up? Thumb keys, foot keys, toggles, etc, go wild.
The linked layout editor (http://www.keyboard-layout-editor.com) allows pretty wild layouts. Try loading the ErgoDox layout. While you might want to go farther still, it does nicely showcase rotated thumb clusters.
The plate & case builder tool can then generate some basic CAD files for a sandwich keyboard build to fit your keys. This can then be laser cut or something.
Because most designs use a Teensy with USB connection, you might even be able to make the keyboard output MIDI, so it could easily plug into a DAW, or whatever.
Hex keyboard, purple keys under thumbs. Extra set of temporary purple (blue) keys 1/2-1/7x, not ratios, applying to one copy only. Two copies of the green keys, one for each hand. Added numbers are 36, 40, 48; unsure about 45. 7, 14, 15, and 30 probably moved. The organized core should be continued as much as possible; I share the sentiment that ease of learning is important. The physical buttons should make fat-fingering convenient, because that's useful for fingering (and for playing lots of extra notes).
The musician probably wants his feet for other things like sustain pedal and effect pedals. I won't be too greedy :)
Though, one possible use of foot pedals is to select the instrument timbre. Since with this layout, you can take the role of two musicians, playing two different timbres, and you might as well have the flexibility to change one instrument out while the other is playing. All your fingers are occupied so it'd be hard to ask them to press extra buttons.
Edit: There are decent guides to designing and hand-wiring keyboards using computer keycaps. I haven't looked far, but something like this https://medium.com/cracked-the-code/a-complete-guide-to-buil... .
The linked layout editor (http://www.keyboard-layout-editor.com) allows pretty wild layouts. Try loading the ErgoDox layout. While you might want to go farther still, it does nicely showcase rotated thumb clusters.
The plate & case builder tool can then generate some basic CAD files for a sandwich keyboard build to fit your keys. This can then be laser cut or something.
Because most designs use a Teensy with USB connection, you might even be able to make the keyboard output MIDI, so it could easily plug into a DAW, or whatever.