You know what, in my head I think I want a whole new messenger.
There's an indexer that acts as a phone book, but at the same time, people can bypass that by directly adding contacts.
All chat history and groups are peer 2 peer and are stored like torrents with the extended backup being self-hostable.
Recent chat history (up to 30 days) can be stored on the indexer, though they're encrypted and so the server is blind to what's in them. They should explicitly be opt-in.
Whenever a user adds a new client (device), all conversations recipients should have to approve in order for them to see the chat history.
It should also have all the bells and whistles, like emoji, stickers, groups, channels, etc.