You could also allow for me to create a new discussion with the same user, that way I can keep banter and production stuff separate while speaking to the same person.
Another item on my wishlist builds off of my first suggestion, the ability to create a PM that is sent to more than one user at once (a la xenforo)
Feedback: I wish the inbox link at the very top, next to my user name, was a dropdown menu, rather than a static link that is highlighted when you have a new item in your inbox.
As for the inbox itself, I would like to request that you can 'opt-in' to show signatures and forum stats (make them separate checkboxes)