I may be the last one to realize this, but I recently noticed that BeOS lets you enter things like ƒ®∑∫ by using the windows key. While it's a really nice feature I was just thinking that it could be expanded fairly easily in Haiku.
For instance, win+letter could enter lowercase Greek letters while shift+win+letter could enter uppercase Greek letters. Or perhaps someone who frequently types in another language might want to use accented letters but stick with a qwerty keyboard layout. So they may want win+e to be é instead of ´. A business person might even want ®¥© on their home row. So customization would also be nice.
What I'm thinking would be a drop-down box for the normal keyboard layout, and for win+letter and win+shift+letter. For example, the default normal key mapping would be QWERTY, but below it there would be an option for Dvorak and international keyboards. The default win+letter would be what it currently is in BeOS and shift+win+letter might be other frequently used symbols. But other options would be available (Science/Math, Business, Accented Letters, etc.). Additionally, for both the normal key map and the win+letter sets there'd be an option for a custom layout where you could remap your keyboard however you want. This also might include functions. Such as making the menu key act like a right-click where the focus is (as in Windows) instead of opening the deskbar or just being useless. An additional feature might be to show an on screen keyboard if the win key or win+shift combination are held down for three seconds so people can see what each letter maps to with that modifier.
Edit: I just noticed, you have to use unicode encoding to see the right characters. Western or other encodings make different symbols appear (although the specific symbols don't matter a whole lot, just examples).