Wow, I've never actually visited your blog before Aeolius, it's pretty sweet. The jellyfish thing is actually really helpful, because I'm thinking about expanding my setting into the Underdeep (the Abyssal zone), and a psionic jellyfish race fits perfectly. I would think that psionics would be more prevalent in deep-sea communities, as the living coral that is essential for storing spells can't survive that deep. I guess I should finish figuring out the more shallows part of the setting, but I can't resist.
Some ideas for core races I had include -
Albino Crabfolk, blind but extremely sensitive and bound spiritually to hydrothermal vents.
Bioluminescent Merfolk, these guys would look weird and semi-translucent, they travel in nomadic tribes, going for weeks without food.
Some Kind of Glowy Fey People, not elves necessarily, but shy, mysterious people with strong powers of telepathy.
Jellyfish Folk, I imagined them being the true masters of psionics in Pelagia, with their own deep-water monasteries and such.
An Odd Race of Scavenging Muck People, all the detritus and decaying bio-matter of the sea floor comes to life somehow in these affable muck-men.
Escaped Slaves of the Aboleth, not Skum necessarily, but odd conglomerations of tentacles, eyeballs and suckers that hunger for freedom and acceptance from the rest of the world.
Most societies are based around vent communities, seamounts, mid-ocean ridges, or the carcasses of colossal sea-beasts. Any place where there is heat, the potential for food, and some kind of natural protection make great settlements. The natural environment is very inhospitable by Shallow standards, but the different races of the Underdeep have adapted nicely. It is the fragility of deep water ecosystems that can make life hard, not to mention the numerous monsters and aberrations that make the Underdeep their home. A game in Pelagia's Underdeep is more about mystery, survival, maybe a little bit of horror as the players stumble across the Ancient Evil Thingies that sleep in the darkness.