[upofadown]

You hardly have to create a new protocol because you don't like Signal. XMPP exists... You can even use the Signal encryption in the form of OMEMO, although it might be a too complicated for good standardization.

>...if you want to support voice or video chat (please don’t), that’ll complicate things even more.

But not very much. If you define a method to securely establish a shared secret then this sort of thing can be an entirely different standard hopefully implemented as a separate binary running in a separate security context.