This is also on the searching of Private Messages thing.
My users (and I) struggle to find messages from or involving a specific user. When searching in:messages, username only pops up if it has been included in the body of the message.
It would be super helpful to have username and name of the message author be searched too, and given priority.
It would be even better to have a specific filter for messages from a user or group when one is looking in their inbox, but I suspect that would be quite a bit more involved!