I apologize if this has been posted before, but anyway...
Perhaps there could be parasties in PS. What they could do is either suck out the health, mana, and so on of our character or suck out the mana, health, and so on of other monsters. Why would we want such negative things? For several reasons. One, promote health care. If you hate this, fine, but parasites are, in my limited experience, not used in that many RPGs if any. Another reason - you are given a way to destroy a powerful monster without actually having to bash its head several times repeatedly with either a club, sword, tornado, cloud, etcetera.
There will be two obvious divisions of the parasite - endoparasite and ectoparasite. The former dwells within its host, the latter dwells on its host. Both types will, of course, be miniscule things that feed off the perhaps unsuspecting host. The host will experience loss of mana, loss of HP, loss of stamina, loss of movement speed, and overall discomfort, among other nasty symptoms. And parasites cannot be destroyed easily, either. While some races may have innate resistance or even immunity to parasites, most will probably not have it.
What is the difference between a monster and a parasite? Well, monsters can easily be taken down hand-to-hand. Parasites need to be destroyed by some other method. Monsters drop loot, parasites may or may not. Parasites can have some special magical properties, though. Parasites can stay around without being noticed, while monsters... cannot.
Pros:
New type of monster
New job or job division
Way to limit uberness, forces players to pay for health services
Realistic
Cons:
Limiting can be annoying
Might be too difficult to implement (tiny creatures are rather difficult to see)
What do you think?