The only thing that really comes close in my mind is some combination of the Signal Protocol (for security/privacy) + IRC (for transport and ease of oeprations) and the Useability of say Signal/iMessages/Messenger/etc – One could argue that Matrix is exactly that, but it isn’t IHMO. I’m not aware it has a mobile first app, it also tries to do too many things and there are no decent implementations that are operationally easy to stand up.