A 6th player with a handful more legal cards, maybe some legal royal apples and the such.
We've added in a 6th player on our own with a custom bag, but by the time the last few turns are being taken, the deck is 90% illegal goods and it's hard to get anything more than 1-2 past the sheriff.
Doesn't a sixth player and quite a bit of extra time? 1 more inspection per round, plus 2 more total rounds? 5 players already slows it down a good amount compared to 4.