While there is a set of official, standardised katakana for transcription of foreign words into Japanese, there are also many informal ad hoc variations floating around, as well as many formally standard kana that have fallen out of use, etc. Some people have even attempted to come up with new kana of thier own to fill in certain gaps for other languages spoken in Japan, such as Ainu and Ryukyuan. I wanted to share some of the interesting stuff I've come across as part of my casual research (kazsurch, if you will) and wanted to see if anyone had anything else to share.
Shimakutuba Orthography seems like a good place to start. The Ryukyuan languages are closely related to Japanese but are languages in thier own right, much like English vs German. Some Ryukyuan languages have more than five vowels, some have the 'th' sound found in English.
