...make and put in waste-based enemies/objects.
I have an achilles heel. I *hate* toilet stuff, especially what they're generally for. If that material is emphasized and made important, It can turn me off from just about anything, including things I otherwise had very strongly liking.
The current "toilet" realm isn't so bad, since it really isn't terribly septic; it's a clean toilet and an underground cave system, and the Clog looks like he's a build-up of fluorine or caulk rather than.... you-know-what. Even if we have to acknowledge what that system may have once been for, it makes sense there's no waste; the system's been out of commission for so long it makes sense all waste material would have just biodegraded by now.
Point of this being right now it's in a good way. Build upon how it is, and please, please spare us squeamish nobody's the displeasure of waste material being added to this so-far-great game.
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If you still need ideas for unique enemies to put in the cave systems, I have suggestions of my own if you'll humor me.