I was thinking of ways to hide DM metadata on nostr and was wondering if it might be useful to have a new type of event which simply wraps a kind 4 event in another layer of encryption (let's call it kind 16). This would enable DM forwarding services to exist that obscure sender and receiver.

It could work like this: a user sends a kind 16 to some middleman service where the encrypted content is a kind 4 event intended for some other recipient that would otherwise be sent on the clear. The service opens the "envelope", sees the original event, re-encrypts it and sends along to the intended recipient (possibly with a delay to prevent correlation based on timing). The client then knows to decrypt the message twice in order to display the content to the user.

If enough people use a service like this it would allow everyone to hide in the crowd. It could even be possible to create "message joins" on nostr where several DMs are forwarded simultaneously to make it even harder to match sender and receiver. Or to add additional hops by recursively wrapping kind 4 (so kind 16 can contain either kind 4 or another kind 16).