[Lord Carnifex]

It won't correspond to any pseudo-scientific theory, but here's probably what I'd do:

Jupiter and Saturn have their own mini planetary systems, with the major moons home to a variety of settings.

Uranus is a bizarre and unsettling place. Its nearly 90 degree axial tilt causes the surface to become baked and frozen in violent seasonal shifts. All these temperature swings have caused the surface to buckle and crack, leading to rocky ground split by chasms and often interrupted by craggy and perilous mountain ranges. The natives look as though they were spawned in some opium nightmare, great winged things crossed between wasps and dragons, ridden by chitinous arachnids, seething with black ichor and hideous intelligence.

Neptune is a world of cold, windswept, stormy oceans and rocky island archipelagos offering some semblance of shelter. Adventures revolve around tall ships and those that prey upon them. In another word: pirates! With perhaps some steam-tech submarines, ala Twenty Thousand Leagues Under the Sea That is, when the natives - foul, squirming things, piscine or squidlike, with mad writhing tentacles - aren't emerging from the depths to attack ships or settlements in the manner of "The Doom that Came to Sarnath" or "The Shadow over Innsmouth".

Pluto is strange and eerie, shrouded in darkness and eternal frosts. It's clear that the world was once inhabited, gloomy temples and blasphemous monuments attest to that. But now, nothing moves but the shadows. Wait. What was that? Did you see someth-