Don't forget Ricochet†, it only does synchronous communication but it does solve the problem of leaking meta-data. All the other clients except Pond leak meta-data.
Maybe in the future when prop224†† is implemented the encryption will be more solid.
† https://media.ccc.de/v/32c3-7322-tor_onion_services_more_use...
†† https://gitweb.torproject.org/user/asn/torspec.git/tree/prop...
That's what's so exciting about the WhatsApp announcement. WhatsApp is by all accounts a pretty great messaging application, and it doesn't just have decent encryption now; it has best in class encryption specifically designed to protect a messaging application, designed by experts who thought about this problem for a long time.
The nice thing about using the onion address (transport layer) is that you have mandatory e2e authentication with only one id that solves multiple real world problems with bgp/dns/tls.
How would you propose to go further from current state-of-the-art WhatsApp to stop leaking meta-data? I know Ricochet is open to use a stronger encryption layer on top of Tor †.