As a Russian, I'm endlessly envious to those who can get by with just one alphabet, even if in different variations — the intersection between these Latin-based alphabets is always large enough for universal keyboard layouts like this to be viable.
I'm really good at typing in the traditional qwerty and йцукен layouts. The problem is that I need two and sometimes the wrong one is active. This is why I envy people who can do everything without needing non-Latin alphabets.
I like the idea of extending Latin with some extra letters as needed to accomodate more languages. Someone would have to make a couple of new fonts probably.