I've seen this topic raised before.
How could editting on MB become more fun?
(And yes opt-in would be a necessity. I have no wish to disturb those who find the current pay-offs to be unsurpassable.)
A monthly ranking of number of edits could be good for the high flyers.
Monthly rank boards for number of edits by category (artist, release, work, series, etc)
Beyond that I'm at a loss.
What can/have others come up with?