Well, it's a bit early, but I felt like bringing up another idea that sprang to mind that got inspired by Metroid. One area I just liked looking at was a lab-created phazon environment where the space pirates were attempting to breed nasty phazon versions of local life forms. Had this dark cave with radioactive blue stuff on the floor and giant mushrooms continuously shedding spores. After clearing out the enemies, the scene was strangely serene while remaining deadly.
So, I'm thinking of trying to capture that feeling with some critters in one of the few Underdarky areas of my setting: They live in underground caves flooded with negative energy, and aren't normally hostile or evil. Think xenobiology, not life-sucking undead.
Background of positive and negative energy in my setting: There's no association between souls and energy polarity. Positive energy is just something your typical biological life form eats up like crazy for generally beneficial ends. Negative energy is the opposite, inhibiting biological functions of us 'normal' critters. The critters in question, though, have adapted in the opposite manner than most, able to utilize negative energy at the cost of being vulnerable to positive energy.
Bonus: Environmental hazard: Help me think of some detrimental effects that'd be appropriate for normal characters traveling through one of these negatively charged regions for too long. Don't want it to be something as quick as 1d6 damage per round or negative levels (though those would be options if they get too close to whatever MacGuffin generates the bad stuff). I'd like it to be something that applies over hours, and takes a long time or exotic methods to fully recover from.