Xoriat, the plane of madness, would be an interesting plane to explore. There could be a quest-chain culminating in a raid in which you fight a higher power of Xoriat that is so dangerous, it utilizes the power of insanity itself to neutralize player characters, and could attempt to banish players from the plane of Xoriat (this way, the Planar boots would have a better use). Additionally, players could have the option of using materials that would protect them from the Insanity effects of Xoriat (this way, players won't have to fight random things such as shoes, boxes, food-themed zombies, and even... mobs of Broccoli that have random effects such as flame shield, chill shield, and could use random attacks/spells such as disintigration [from it's eyes!]).
This would not only allow for more quests at higher levels (as this could be a level 18-20 pack), but would also mix things up a bit so that it doesn't feel like doing more of the same.
Thoughts anyone?