[ad8e]

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.

[mtsr]

Hex keycaps are definitely less common, but can still be found (https://www.littlekeyboards.com/products/hex-keycaps). Or you could get 3d prints of the exact profile you want.

Edit: Interestingly, above were also the result of an experimental musical instrument. See https://kbd.news/0xC-0x33-1525.html