Forward secrecy (possibly mostly covered by short-term keys + re-encryption using long-term symmetric keys, but might still be worth discussing separately, to see if we want a more "proper" solution like double ratcheting)
Key verification, e.g. a better alternative to manually verifying fingerprints. This topic could include using QR codes or similar, or even something like Key Transparency to automatically verify keys (which Aron presented on at IETF 113).