Signal or something that is a direct derivative of Signal, and can be implemented by anyone other than carriers... Almost like XMPP but I guess not XMPP? XMPP with Signal for E2E somehow?
That's a well developed thing already:
* https://en.wikipedia.org/wiki/OMEMO
Common OMEMO supporting XMPP clients are Conversations[1] for Android, ChatSecure[2] for iOS and Gajim[3] for the desktop.
[1] https://en.wikipedia.org/wiki/Conversations_(software)
[2] https://en.wikipedia.org/wiki/ChatSecure
[3] https://en.wikipedia.org/wiki/Gajim
You can see the OMEMO status of the various XMPP clients here:
But they don't federate. Which I think is a pretty clear example of why not having one true solution isn't a technical problem, it's a political problem. Companies don't want to make it easy to compete with their products.