You can stop posting suggestions now. The numpad - and + have been decided on and it is not as if we have much of a choice. This is not about keys that are NOT being used but about keys that can be caught by an OnControlKeyEvent. And believe me, there aren't that many.
There is another option though: OnDefaultKeyCaught, which can capture any key (no, not the "any" key

). But using this would require a major rewrite of flymode. And it is also very unreliable with non-US (even UK) keyboards.