I've always wanted something like the space cadet keyboard with a couple of extra modifier keys vs. what's typically available today for typing unusual characters (and the keycaps for making that easy).
https://upload.wikimedia.org/wikipedia/commons/4/47/Space-ca...
Print your own keycap labels with Avery sheets for starters, and once you've got the layout nailed down, you can send off to have custom caps made by whatever process you like.
I have.
Keyboard maps are on the host, not on the keyboard. Engravings on the keytops are meaningless and are not what determine how a key is understood. A (for example) U.K. 105-key keyboard and a French 105-key keyboard just look like two 105-key keyboards with all the same keys to the host.
Several common operating systems just combine all keyboard input from multiple plugged-in keyboards into one giant "union" keyboard and apply a single keyboard map to it.
I've been working on Linux/BSD software that allows individual different keyboard maps per keyboard, for my user-space virtual terminal system. It's achievable, but I've not encountered anyone else who has seriously attempted to make such a thing work, in the general case where arbitrary USB keyboards can be plugged in and out at runtime.
It's been a really low-priority desire (I won't dignify it with "need") and so I haven't even really worked out a layout or even carefully defined what I'd like, but yeah, the Really Big Keyboard would fill at least a small need.