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...

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.