Ctrl-[ is impossible to type on, say, a German layout, as the [ character is hidden behind AltGr + 8, where AltGr is the right Alt key, which is itself (for most terminals/systems) just the combination of Ctrl+(left)Alt. So you'd basically have to type Ctrl+(Ctrl+Alt)+8 which is not possible....
I remember reading something, or maybe it was a talk on some conference, by someone who had some deep knowledge of the terminal layer. He explained some ways for alternatives to, say, ctrl+i, because something about the way the control characters only use 5 bits and if you type ctrl + a key that has the same lower 5 bits as "i", that also works like ctrl-i or something. I've been trying to find that video (or web page) again but I couldn't. But I think back then I found a replacement for ctrl+[ but I have long since forgotten it ...