Interesting idea. Would the messsges be sent peer to peer then or would they still be communicated via relay? Personally I worry though about things that cause lock in to specific clients (there's a kind of centralization risk for nostr there too). The idea of using a different key for DMs does seem useful, maybe even one that can be rotated to give forward privacy. I was working on a NIP last week for a type of onion routing for DMs and there it was suggested to use something like RSA keys. Reason being that messages can be decrypted without knowing who encrypted them and this ends up improving privacy under that scheme:

GitHub

NIP-103: Onion Routed Direct Messages by threeseries · Pull Request #499 · nostr-protocol/nips

This NIP should help improve privacy with respect to kind 4 events on nostr.