I think there are two things here:
Should we use URI's for never things and try to limit @, $ and & for the classic format. This thread I think is a good example of why what would be a good idea :) I would like that the libraries we use like ssb-ref hides away this complexity if we do go down that route.
How do we handle things that don't neatly fit into our message, feeds, blob world. Private groups, p.o. boxes and fusion identities come to mind. Right now a private group is a message type in TFK/BFE. What is a p.o. box even? It's not really an identity, it's more like a key pair. I was having a conversation today with keks where this came up and thinking about how we categorise them will help clarify and communicate what they are.