Aside from the typical encryption worries here are some features I would love to see being offered on a alternative messenger app -ability to give out your alias or username to people to reach you on whatsapp instead of a phone number, something you can change later after banning the contact. -transcribe voice messages to enable search among them. -ability to hide the fact that i have listened to their voice message.…
Implementing this could make key management either less secure, or more difficult for users. If the user still had access to both devices, it wouldn’t be too bad. But if you want the users to be able to ‘recover’ their message history on any device, then you’d need to use something like the mnemonic seed phrase, which isn’t an improvement in UX, or security.
The audio transcribing sounds difficult too, as you’d have to do it all client side if you wanted to preserve the security model.