I guess cryptographic forward & backward secrecy would be the most important addition, along with hiding the recipient metadata. I see Jeff started double ratchet at https://github.com/nostr-protocol/nips/pull/1206 and moved on to MLS. I'll take a look.
I think forward and backward secrecy are unachievable in Nostr. You can either be able to load your DMs in many clients and not have any type of real forward and backward secrecy OR you can have DMs that are only visible in the originating client, and thus "broken UXs" everywhere else and then yes with forward and backward secrecy.
There is no way to do both.
Maybe we can have something like Signal for connecting sessions on different devices / clients so they see the same messages.
Then you have just broken forward and backward secrecy: Attackers can use that feature to reassemble the ratchet state and decrypt all your messages, past and future.
