I much prefer original ideas, but I see this one as almost "necessary" due to the 6.5 changes where you can allow a maximum number of Private Messages in any give users' inbox.
The idea being this showing up top of the Received Messages pages (viewmessages.php):
Inbox: 58% full
(50 maximum Private Messages allowed)
And yes, this is a pretty simple thing, but I am already getting a number of my members hitting their "max" point all the time and unsure of how many they are allowed to retain at any given time.
This will eventually segueway into having "saved PMs" as well - but that is a topic for a later date.