Magic is the lifeforce of all living creatures, and it is from it that all life is made - in areas with lots of undead, magic might actually be missing or weak, which puts a whole new spin on how dangerous the undead are. Even simple zombies can be troublesome if you're relying on mages and you hit a pocket of undead making a low-magic zone...
This really comes from an ancient Chinese novel. Imagine during the PCs' travels, they come to a place where the gender roles are completely reversed. What adventures will the PCs encounter in such a place?