Imagine this: Mastodon now ships directly with a built-in Matrix server, all direct messages (DMs) are end-to-end encrypted and the DM interface is completely overhauled to look similar to Twitter. When you create a Mastodon account you also create a Matrix account with the same ID (ErikUden@mastodon.de ⇒ ErikUden:mastodon.de).

If a Fediverse server deletes this functionality, isn't updated, or you're messaging someone on an ActivityPub service that doesn't have it implemented, a DM just reverts to the old, unencrypted version, still through the new interface whilst notifying the user that this DM isn't encrypted.

Users can login to their Matrix account through any third party service using the same credentials as for their Mastodon account and don't have to use it only through the Mastodon web-interface, but it's optional on the new DM interface to only display DMs / group chats from Mastodon users.

Users outside of Mastodon can DM users with any Matrix account, tied to a Mastodon server or not.

Why re-invent direct messaging when it has already been done so well with Matrix or XMPP?

The reason I thought of this was because I found it so neat of Pleroma to use WebRTC for direct messages, especially with the often better interface. The visibility setting of posts is one solution, which I think has its place even with this new DM feature I imagined, I just find it unintuitive.

The reason I wouldn't just implement what Pleroma already has is because it is unencrypted, and before you invent a new standard, why not just integrate a federated messenger into the federated social network?

What do you think of this? What problems would this new system bring and would you be in favor of it?