I've felt a little creative this morning. I guess it happens when I lack sleep or when I've just been doing a lot of writing. The new Old World cards are supposed to make the game feel a little more cooperative, but I think that was just a throwaway draw. It didn't really happen. We're all still dicks. My suggestion doesn't make the game much more cooperative, but I think it's much more interesting. What if a 6th player actually controls how the Old World functions?
He'd have 6 power points to use and even some figures (no cards). The Old World deck wouldn't be set, but tracked for the appropriate number of turns, modified accordingly with Franz's Decree. During the Old World phase, he can spend any number of those points to look at that many more Old World cards from the top of the full deck and choose one of them to apply. The rest are put on the bottom of the deck. Any that get used up during the course of the game will get discarded forever; no possibility of reusing Franz's Decree, Electors Sue for Peace, etc. The figures will all be 0/1 with effects. The "cultist" costs 2 points and will remove one corruption from the region during the corruption phase, after checking for ruination. The "warrior" substitute will also cost 2 and simply act like any normal hero token, except that Slaanesh cannot directly benefit from his presence. The "greater demon" costs 3 and does both.
Effectively, this player would win if nobody else does. This is already the case anyway, right? There are no ties with this game. The Old World is already considered the winner if everyone else loses, so that is unchanged. The difference is that this player only wins if the game lasts as long as possible. He can't rush the win as others could when left unchecked. It seems like this would be balanced, assuming I could figure out the appropriate number of each figure. Undoubtedly it would make the game harder, though Slaanesh might actually profit. Slaanesh is fine advancing his threat dial at a leisurely pace and still getting the win. The average number of rounds would increase, making this a slightly more comfortable scenario for him. That makes Slaanesh a good character to hit by default. Of course, that easily changes once someone gets a significant lead.
Feelings? I'm going to propose this with my group soon to try it.