I voted for keep the spot empty.
Quite simply because i'd rather play in a small group than a large one.
While the idea of a cameo spot is cool, i have a feeling it will be full all the time, all be it by different people.
If this sopt were filled occasionally by a key 'npc' fleshed out by a real player, this would be a cool idea.
But i think just a random sixth person every month would be an excersise in fail.
patty's 2 cents