I'd love to see end-to-end encrypted chats in openbook.
Of course, it might be hard implementing it in a way that users can have access to all of their chats from any device.
But Wire messenger for example, has found a way of "multiple-end-to-multiple-end" encryption where all E2EE chats are visible from all devices. You might want to look into the details of this open source app :)
The feature of ME2ME-encrypted conversations would make me chat with my friends on openbook too; Otherwise I'd rather stay with my E2EE messenger.