There's already a forced swimming one, it's called Deluge. I don't think it's used very often. And what the heck is the prune thing about? You're deliberately being silly, right?
Also as Silvanus says, plague != taint. Rats have nothing to do with it.

forest demesne rooms-herbs grow faster
geo demesne rooms-rats appear faster
aqua demesne rooms- can be used to breed turtles in! (I couldnt think of anything, hehe)

dunno really, just thinking

Mutant rats!

I think the post-demesne hibernation resistance is better than rats. By a long way. =/

Geomancers- The land begins to speak in your mind, telling you bad things. No bad effects, just a little rp as simply being around the Tainted land advertly effects the mind. Like seeing things that aren't there, hearing whispers and screams, that sort of thing.