Forward secrecy requires every message to be encrypted under a different key that is not derivable from long term secrets, which probably means they have to be randomly generated and both parties probably need to remember those keys somehow, forever, in order to read old messages. We might need a new event kind that stores all those keys encrypted under your own key.
Discussion
But if people must be able to read their old messages, than compromise of a person's nostr key compromises all of their past communications with everybody, and all of those everybodys should be concerned about that (your secret message exposed because someone else was careless), and forward secrecy won't help.