Perhaps a nation buried deep in the desert, a small population of hardened survivors. These survivors, having had to survive so long in such small numbers, have mastered the craft of life, creating soldiers and workers from the very sands that once constrained them.
A bit of a mix between your sand nation and golemancy.
Also, an idea that comes to mind is a tribe of immortals in the mountains- Legendary monks and sages who have been reborn thousands of times over the years.
A tribe of men who worship, and have a direct relation with dragons.
As far as water goes, rather than a cliche Atlantis setting, we could have a group of albino cave dwellers, worshiping a dark monster who lurks in their aquifer.
A city high above the clouds, a sanctuary for all those who require help
I dunno, just a few ideas. I love the ice witch and fire nation ideas.
I forget exactly how deep I had gone in developing the Titans, but in the end, I had a bit of an idea.
Perhaps, for there purposes, the Titans could be put into the lore. Essentially, all massively powerful brothers, the Titans were the first born upon the land. Upon the command of (Whoever the hell created this world), They built palaces and cities in their image, metropolises that sprawled across the land. In the midst of each of these great tributes, was a totem. A single structure standing out in the middle of their masterpieces. Within the Totems, was imbued a fraction of their power, something to keep the cities alive and thriving.
But eventually, bickering broke out among the brothers. A great war was waged, leaving behind rubble where great cities once laid. (Previously mentioned powerful guy) saw the havoc, and put an end to it, saving his world from their wrath. He populated the world with weaker beings- Those who could appreciate the planet, but not quite powerful enough to do what the titans had done. Now locked deep beneath the world in near impenetrable vaults, (Jesus-Man) believed that they had lost their grip on the world (which needs a name, because I am tired of writing 'the world'), but that wasn't entirely true. The Totems left behind were soon taken a hold of by man. Channeling the power within, they were able to give themselves great magical power and might. Ever since, humanity has been working tirelessly, building up nations almost comparable to the great works of the Titans, growing to be a more powerful race every day.
Do you hate it? Do you love it? Just wanna throw out more ideas.
God it feels good to write again.