Anything apart from the two mainstream default harmonicas (Solo-tuned fully-valved chromatic, and un-valved Richter 10-hole diatonic). Alternate tunings, different construction, new functionality, interesting old designs, wishful-thinking... whatever!
Karl: I like how your b/w diagrams can pack a lot of useful information into a tight, elegant format. Compared to a more expanded and colorized representation, it requires a little more theoretical knowledge on the part of the reader. But with the right audiences that knowledge can be expected and is totally appropriate. Bravo!