@alex ah, this looks interesting. Allow me some time to read more about this.
nostr aside, it seems to me that while some features could just be layered on top of ActivityPub, it would just require that multiple projects mutually violate the spec and that there be translation layers for those that don't/won't.
Targeting the lowest common denominator (mastodon) is frustrating, especially when we're talking about things beyond just schema interpretation (e.g. transport layer changes).