living off trace amounts of plankton floating in the atmosphere
Besides geothermal energy (which I doubt could support much of an ecosystem), there's nutrients from aboveground. Mud getting washed down from cave rivers, any animals that live around a cave access point, that sort of thing. And then there's magic. Some blatantly impossible creatures don't make any biological sense, and thus don't have to care about normal rules of ecology. Like the eye beasts that have no mouths and are said to subsist on evil. I can't think of any other existing examples, although they might exist. In any case, coming up with new ones is trivial.
Luminescent fungi that don't need any energy to fuel the luminescence. Mysterious glowing crystals. An animal that eats "energy", meaning that it can convert geothermal heat directly into biomass, and doesn't have to care about how mass-energy equivalence actually works. Animals that just don't have to eat to live and grow. Even though this game is realistic, it still operates on fantasy physics that don't make much sense if you look at them closely.
Also, it'd be cool to have caves that are connected to the ocean, and have whales in them. Or cave whales. Or anti-gravity cave whales, that swim deep in the undertunnels, but occasionally fall upwards towards the more inhabited areas.