Skyrim makes me extremely reluctant to have a bunch of dragon worshippers. That's probably now more cliche than the stereotypical fire, water, earth, and air nations.
I also thought of a nation of cave dwellers (in my case, one that might use shadow magic) but I didn't add it to the list. The issue with it and the sky-city nation is just the difficulty a Keeper would have in counquering them. Also, how would we fill their portion of the map? Would we just have half the map a featureless plain with all the interesting stuff above or below?
I do like your idea of the desert nation, and we could possibly work with the Titans as the main plot.
While we're on the topic of plots that we had planned out, I guess I'll share where Shaige and the Paterdomus plot was headed. The Keeper was going to do the impossible and assemble a horde of unrivaled size, consisting of demon, monster, undead, and man alike. With hundreds of thousands of followers he would cleave a bloody wound through the realm of Paterdomus as he marched to their capital, easily winnig every battle on the way and sacking everything he encoutered.
He would use a tactic invented by the Mongols; having a massive vanguard driving forward thousands of terrified peasants from the countryside and razed cities, killing those that fell too far behind. In this way any army that tried to stop him would eiter be trampled beneath a horde of peasants, or at best would kill thousands of their innocent countrymen before the real battle even began. He would continue in this fashion until his army an the peasants were a few days' march from Paterdomus, then he would order all the peasants marched to the banks of the Suricrove and massacred.
The giant river would literally be stained red from all the blood, and this would serve two purposes. Firstly, the water priests' failing enchantment would be undone, allowing the ice witches and their northern armies to join Shaige's ranks. Secondly, it would spell utter doom for the entire city of Paterdomus.
Shaige's army would march within sight of Paterdomus' walls. The grim defenders would prepare to fight to the death, only to be confused as the vast army stopped just out of range of any mages or trebuchets on the wall. One cloaked and hunched over man would advance, the defenders understandably mistaking him for some sort of emissary rather than the scourge that was their ruination. So he would have approached the walls unchallenged, and in the blink of an eye performed a god-like feat of black magic.
Empowered by the tens of thousands of souls that he reaped as the peasants were massacred, the Keeper used blood magic to lift the entire bloodied Suri river into the air, leaving only a parched riverbed. The water would hang suspended over the city for a split second before crashing down in a mighty deluge, the equivalent of several feet of rain in seconds. With half the defenders swept off the walls and the people cowering inside the city literally drowning in blood, there would be no resistance for the massive horde that stormed the cit and began pillaging it.
The Keeper would take some time to bask in his triumph. Then, in his arrogance, Shaige would walk unchallenged through the city, reveling in the utter mayhem. Intending to personally sieze the citadel for himself and depose of the pathetic Prophet of Caldor that had seemingly abandoned his people to cower beneath the temple, Shaige's plan would utterly fall apart. Out of nowhere the ground would tremble and plumes of fire would tear the citadel asunder. Its impregnable walls would buckle, collapse, and melt as Caldor himself clambered up from below, summoned by the Prophet. Caldor's heat and radiance alone would incinerate half the city. The combination of thousands of the marauders suddenly dying with the sudden arrival of a colossal, magmatic humanoid that is considered a god would be more than enough to send Shaige's army into a frenzied retreat.
Feeling victory slip from between his fingers, Shaige would become emboldened and reckless. Greater fires only cast greater shadows, so Shaige would take the form of an equally colossal wraith and have a dramatic battle with the fire god, before eventually being impaled upon Caldor's infernal sword. The writhing, living flames that constituted the blade would tear at and devour the layer of shadows that cloaked the wraith, revealing an unholy skeleton with hellish smoke, blood, and wailing souls magically trapped with the ribcage. And then Shaige would be engulfed by fire and be promptly burned to death, not even his supposedly immortal soul being spared by the divine fire. With his dying breath, the great spirit would utter a curse so vile and filled with hate that to hear it would be death, though fortunately the sound of the raging inferno would overpower his curse, no doubt sparing thousands of fleeing demons and monsters. Caldor would be wracked in pain as he fell to the ground, consumed from the inside out by horrible magic. The blazing leviathan would smolder on the ground for only a few moments as every shadow in the city moved to choke out the flames' dying gasps for air. In the end, Caldor would be reduced to one miniscule ember, cursed to never go cold so as to suffer for eternity knowing that it had once been the god of fire.
From there, the massive army that Shaige had miraculously brought together would of course splinter into a hundred factions. Dozens of the most powerful minions would claim their fallen master's throne and amass followings in an attempt to kill the other contenders and rule the ruins of Paterdomus and the lands beyond. However a large fraction of the horde would simply flee, left confused and terrified by the death of their overlord, the only entity that would ever be able to maintain order with such a diverse and large group of followers. In the end, many of the remnants of Shaige's army would find their way to join forces with either the Carver or the Ripper, whichever side I thought to be less likely to win.