I'd pay money for one of these, if it had the keys which I want, which this one doesn't. I think the trick would be to somehow make it customizable, but I'm not sure the best way to do that. Parametrized PCB design? 3d printing?

I'm not quite sure, but my dream would be smaller buttons, coarsely modelled on the HP48-style keypad. I can't imagine many people share that dream, but I've wanted to make that for a long, long time.

Coincidentally, you get a long ways there by simple adding a Greek keyboard layout. You hit meta-space, and ασδζχψ. meta-space, and back to normal.

Foreign-language keyboard under GNU/Linux also have a layer with many convenient math keys (²·≈·¾ all exist natively in my normal layout).

A smaller win for me would be to simply print all of that on my keycaps.