Another idea: true sub-accounts, that is, npubs, just like new addresses under Bitcoin. Allows for one to interact more anonymously without creating an alternate key and maintain interoperability, etc.

Additionally, multiple personae with one key would allow others to follow one persona under a key but not all, just as a personal and professional side to an npub. More basic* than tagging.

This may prove difficult, but I don't know, since I'm not a cryptographer.

*basic to mean low-level or foundational, not simple.