How about making all keys in a sheet of molded clear transparent plastic, along with a cheap resistive-touch e-paper screen placed underneath the keys, and some led-lighting around it.

That would give a physical keyboard with fully configurable key labels.

The only permanent decision will be the dimensions of the matrix of physical keys, i.e. 3, 4 or 5 rows times 11, 12, 13 or 14 keys, possibly with an extra large key on the bottom row to potentially use for space. If you want cursor keys, sym, ctrl, tab, Fn, esc, alt or any other modifiers will be a user decision depending on the keyboard layout you download, install and display on your keys.

I for one will have a layout that includes pipe, ampersand and all sorts of braces easily accessible :-)