What if there were relays that only handled encrypted DMs? You could write your DMs to it, but only read DMs after you prove you have the right private key. Would help address some concerns people have over metadata leakage.
They could even follow a policy of deleting DMs after some expiration date, so you aren't taking on as much risk when sharing private information in messages.
A messenger app, and specific messaging relays for it to use is what you would need. All of the current clients attempt to write events to every relay on their list. You'd need your private DMs to only go to relays which handle them correctly.
NIP-26 offers another option with key delegation. Use disposable pubkeys for every message. That way the metadata only shows messages being sent from one random unidentifiable npub to another. But the holder of the private key knows which npubs to fetch messages for.
