Would something like an HD wallet fix this? Say I want to DM you. I send you a message and part of the message is proof that I have the key that controls my public-facing Nostr account, and part of the message is a new npub that only exists for the purposes of my conversation with you. If you respond to my message to you, you send the message to that npub, from a new npub of your own and proof that you have the key that controls your public-facing npub. Now we are both chatting using npubs that only you and I know about. The only thing anyone else can see is that you received a message from an unknown sender. No one knows who that message was from or even if you responded.