Right...So at year 0, all these various civs suddenly *FLOOF* appear out of thin air. Maybe it's like Civilization, and the history-less wandering nomads spontaneously unite behind their own leaders. Maybe it's like Unicorn Jelly, and they arrive through dimensional gates and start adapting to the new world. Either way it works alright.
But what happened before all that? Well, we know that the Powers already existed, and the megabeasts/semimegabeasts already existed. Were there other races before our five main ones? Did the Powers have civilizations? Were the Gods more active?
Lotsa people seem to ask for things like old catacombs as HFS under plains or something. It might be neat, but who would have built them? Any cities or civilizations would have been long-dead ruins by now, but they still might be fun to find...
Dealing with only humans, elves, goblins and kobolds for decades would suddenly make things much more interesting if you broke into catacombs with pictures of lizardmen on the walls. With the occasional weapons made out of some unknown material that is no longer forgeable even by dwarvenkind...a little better than steel, not nearly so good as adamantine.
In any case, I'm thinking it might be fun to let ancient and dead civilizations sprawl across the map and then all die out, long long before the modern day arrives. It could be greatly simplified and sped up--Less hist fig data might be required. Stories and legends may, perhaps, never even be translatable. Run it ten years at a time, because it's all fuzzy anyway.
Just think it might be entertaining...even more so if the landscape itself can change over time (growth of forests, deforestation, the creation of mountains by the Powers). But this is a long-distant power goal.