Whistled languages seem like a useful idea, but I can't find enough material to to anything more than say "oh, that's a neat thing that exists". In particular, many whistled languages seem to be transliterated (I know that's not really the right word but close enough) from "normal" spoken/verbalized languages with a loose mapping of sounds to whistles, and ever since I encountered them I've wondered if the same could be done for English. Unfortunately, there's not a lot of good material online describing how the sound mapping works, and what little there is assumes that you're a linguistic expert, so it just sits in the back of my mind as an interesting idea with no (good) path to realize it.