Well I'm an Emacs user and would love to have Apple's butterfly mechanism style keyboard with the existing modifier keys for macOS (Command, Option & Control) plus the modifier keys from the Symbolics keyboards (Meta, Super & Hyper). That way I can separate out the key chords for window, application, and system commands from the key chords for Emacs text editing commands. I'd pay at least $1000 to have it custom built.

I think some of the variables include key layout, key switch type, keycap shape & material, key dimensions, the material of the keyboard itself, type of connector, LED colors & placement, extras like auxiliary displays, knobs, switches, sliders, and touchpads. I could continue...