I imagine that we could have relays less responsible about content distributed if they saw ciphertext.
That be achived if users could publish a symmetric key together with their npub; the user would post notes with his symmetric key, the key could be rotated, and published at will (website, a keyupdate note, ...).
Replies could be ciphered with multiple keys: the replied's and replier's.

Relays could then see only ciphertext passing by.
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