It doesn't have to be a timed event, but maybe a questline in game that a player gets to design, with even a quest reward they get to come up with as well.
We've seen the amazing type of stuff that people came up with for the Tank/Heal/DPS class glamour stuff, and I think this would be just as popular, if not a bit more so.
People would have to be able to keep it within the lore of the game, possibly 'stretch' things a bit to make things work within the world, and I'd like to see what kind of stuff players would come up with.