A story at Hack-A-Day by Lewin Day about a bloke who goes by “SzurkeEg,” making a clutch type derailleur for 6-9 speed rear cassette systems. This gives me hope for the pile of dead derailleurs I have.
https://hackaday.com/2019/08/13/mix-and-match-parts-to-build-a-better-mountain-bike-derailleur/
BigGear's Blog 