This conceptual campaign space came to me in the border between sleep and wakefulness one night, and stuck around enough that I'm going to share it all with you.
The simplest way to explain this setting is that water is everything. It is the source of all life, pure magic and the conduit between worlds. Literally!
What do I mean 'source of life?' Obviously the same rules apply there as elsewhere in the conceptual universe, if you don't drink it or stuff with it, you die.
Pure Magic? Now we're into the meat that sets this place apart. Water in this setting is a spiritual essence, the miracle that sustains life and lets those able and willing to perform miraculous feats: magic by any name. Those versed in its control can draw upon the wellspring within and the waters without to create great effects.
I suppose you want a tighter explanation, yes? This setting does away with vancian casting and instead uses a spellpoint pool, like psionics. All spells are drawn from water within you, barring certain rituals near or in bodies of water. Using up spell points dehydrates you, eventually inflicting dessication damage and exhausting you as your lack of water adversely affects everything about you. Obviously the only cure is drink more water. Most folk live near any permanent body of water, where life and civilization through magic flows. Adventurers like you forage out into the wastes and oddlands where water seldom, if ever flows.
In this setting, the Sun is the enemy, the destroyer of water, the burning agony, whose fury drives the soulless beasts in the depths of the desert into fertile land and scours the world with great storms and heatwaves. Undead and worse here are aligned with the sun, not the night, and are beings of fire and ash.
It is the moon, gentle and pure, the rainmaker, the misty fog, the guide to the heavens that succors this world. The moon waxes and wanes, but never less than half, and at full shines so bright that the sky is tinted silver. The moon is held to be the source of all water, and the center of the Astral sea. Peoples pray to it, fight in its name, emblaze moonbursts on their armor and place moon crests on their buildings. The moon rises when the sun sets, sometimes before, sometimes after.
Conduit between worlds is no exaggeration. At night, when the moon is fullest and water washes over the lands, the ocean, great lakes and many ponds literally stream into the skies like auroras, great streamers of water rising upwards to their creator, opening the conduits to the Astral Sea and other worlds within it (planes).
The Astral Sea is literally that, a vast network, tiered, of flying rivers and pools and eventually a single ocean at its apex, closest to the moon, that links the various other worlds together with this one, home to stars, shining objects of wonder drifting within it, along with innumerable islands and phenomena the likes of which those living on the Primus World have never seen before.
Like Iron Kingdoms, this changes the core game (in this case pathfinder) enough to mandate having the campaign sourcebook, since every class is pretty much revamped to suit the setting.
Major Mechanics changes off the top of my head (a bunch of QoL and/or 'make the game go faster/add social aspects beyond succeed or fail checks' stuff too but that's for a different forum) -
Magic is massively revamped into a spellpoint based system. Wisdom determines your bonus spellpoints for everyone, another mental stat determines DC's. You can get magical focus (works like psionic focus) to use metamagic feats.
Running halfway through spellpoints fatigues you as you dehydrate (similarly, failing saves against exposure to arid areas will start draining your spell points), running 3/4 exhausts you and you start taking dessication damage until you get a drink.
-Create Water is 9th level here, and some consider that spell the dominion of the gods. As well as other lower level water making spells like wall of ice, waterspout, etc. are all 9th or gone or else folded into Create Water's effects considering by RAW you can get infinite spellcasting with them if they allow you to drink. Controlling it is still the same level though.
Uses a Magic rating system, every class and monster has a magic rating just like it does a BAB. A number of cheatyface spells like Endure Elements might be higher level too.
-Obviously all different classes, and yes there are options if you want to be the enemy of life and follow/bring the Sun.
-Since water is life, and also magic, pretty much everyone can use at least a bit of spot on the wall magic, even 'mundanes'.
Major Race changes
Merfolk replace elves as the mystical race and have bonuses to being casters. Obviously even though they live in water they don't get ridiculous magic advantages from it, at least not at first. Noteable for being able to reach the Astral Sea easily without need of magic, but require assistance to move around on land.
Gillmen replace half elves as the borderline mystic race, and have to use special equipment to adventure away from water for long periods. Noteable mainly for their charm magic, but dangerously dependant on water.
Humans replace... well themselves, these ones have a great deal of endurance instead of 'flexibility to be the best race' and resist moderate environmental conditions much better, but fare poorly in extreme conditions.
Suli replace half orcs as the other borderline brawler race (many Djinn are sun aligned too!). Noteable for being strong and magic resistant, but by the same token utilize the finer arts of magic poorly.
Dwarves are more or less unchanged, save that these ones dug into the ground to escape the sun and are closer to Fremen then Vikings. Noteable for being itinerant and exploratory, but also extremely bad swimmers who handle large amounts of water poorly.
Grippli replace Gnomes as the quirky small race, with an affinity towards swamps, the most fertile display of the moon's power. Noteable for being clever and fast, but by the same token flighty and physically weak.
Halflings are also more or less unchanged in their entirety. Still good at stealing, and good at getting into trouble for stealing, whilst being bros.
The Campaign arc typically starts off either on a lakeshore, riverside or wetland (at any rate, players playing merfolk shouldn't be penalized if they want ot use their swimming advantage) and gradually move away from major sources of water into arid deserts and badlands further and further away, then sojurning through the astral sea to adventures in other planets.
The overall theme of the setting is there isn't one yet, parts of Nevada desert wild west, arabia, the gobi desert and interior australia (as well as associate cultures) informed the sun aspect while thailand, the amazon basin, the nile delta and the great lakes (and associate culture) informed the moon aspect of the campaign setting.
Anyways that's my preliminary outline of the campaign setting. What do you guys like, hate, disagree with or want to see improved or refined, if any?