This will work once we have keycaps with e-ink or similar technology and this will be super cheap for everyone. Then we all will have keyboards with infinite number of characters and new chapter of typing will begin.
Another idea: let the Windows-key (I have no use for it anyway) trigger the appearance of an onscreen palette of glyphs, which you can click with the mouse.
Windows + . (dot key) does that by default. Sure, most of the symbols are just weird cat faces or whatever, but the basic feature is there.
I'm on Ubuntu :)
Doesn't something like that already exist: Lenovo's Yoga Book C930? It had 2 screens, with one being an e-Ink keyboard. Very useful tech.