Unsolicited giftwrap DMs could be ignored except for “chat requests” which is a giftwrap of that users device pubkeys and inbox relays. This would get rid of the remaining receiver metadata on giftwraps, and enable “device keys”, where each DM gets encrypted to each device. If your root key leaks, the messages are still protected.
Cool work being done in this area by Vitor Pamplona (npub1gcx…nj5z) and fiatjaf (npub180c…h6w6) :
https://github.com/nostr-protocol/nips/pull/1306
https://github.com/nostr-protocol/nips/pull/1647