Quick reply...
I like that you make the keyboard QML based, with large keys, with arrows, quite transparent and not opaque, and the layout is all written down in QML.

As I understand, there are three free unassigned keys in the keyboard. It could allow for switching between four keyboards, at least. You could make the C++ part of the code able to switch between four different QMLs, make the three keys send corresponding signals to the C++ part of the code, create four nearly identical English layouts, and then let users customise keys and keycodes of the keyboard layouts to their hearts' content.
Like:
one layout English,
one layout special symbols,
one layout Greek for mathematical formulas,
one layout Esperanto.

What do you think about it?