Every campaign needs Atlantis. Or Rome. Better yet, why not combine the two? That's what I did in creating Hyperborean Thule, but let's back up a little bit.
One of the many things people complain about in D&D is the way that its magic is so codified, with spells assigned to levels. Indeed, the very concept of spell levels strikes many as a metagame concept that's found its way into the implied setting. I think it's a fair criticism, but it doesn't bother me anymore than do level titles (which I love, by the way). Looking at both of these things without any preconceptions, the first idea that suggests itself to me is that there must have been some ancient civilization that was really into systematizing and organizing things and both the arrangement of spells and level titles are watered-down holdovers from these old systems, like the way that late Roman military ranks morphed into noble titles over the centuries or how the dress of Imperial Roman courtiers was adopted and adapted by the ecclesiastical hierarchy.
So, in my campaign setting, the Thulians are my "Romans" whose civilization laid the groundwork for the world that came after them. Actually, the Thulians are a bunch of different archetypes. Besides being the Romans when it comes to systematization, they're also the Atlanteans, a mighty civilization whose command of both magic and technology was rivaled that of the Eld, whom they overthrew and from whom they learned a thing or two. The Thulians are also my "good guys gone bad." Originally a remote barbaric tribe from an island to the north -- Hyperborean Thule -- they eventually became the nucleus of a revolt that cast off the Eldritch yoke and held back Chaos for centuries before finally becoming corrupted by it. The Thulians in turn fell prey to revolts, as well as the punishment of the gods, who sank Thule beneath the waves rather than see it become a toehold of the Abyss in the world.
Before the end, though, the Thulians did all the things you'd expect magical Romans to do: built roads, founded cities, researched spells, created artifacts, established laws, collected the gods into a single pantheon, and so on and so forth. This gives me an excuse for any systematic elements of my setting -- I can claim Thulian antecedents -- while the distance in time between the sinking of Thule and the present gives me lots of leeway to change anything I want.
Plus, let's face it: with all the ruins lying around in a typical D&D world, you need magical Romans to explain their existence. The Thulians are mine.