While having a conversation with an old gaming buddy yesterday, I stumbled upon an answer to my previous issue. This is only a solution for Ilion, but since I conceive of it as the equivalent to the Wilderlands' Campaign Map 1, I'm ok with that for now.
Ilion is a fairly prosperous city due to trade and the crazy primordial jungle fruits that its farmers produce. This prosperity means that Ilion has the means to make colonies of a sort. Scattered about neighboring islands and in areas further inland are small forts designed to accommodate various settlers. Homesteads and villages are there too but in much smaller numbers. At this stage in Ilion's colonial project, the areas outside the forts are far to dangerous to allow for permanent settlement, even with the garrisons' ranging occasionally into the frontier.
I like this set up for several reasons. First, it kinda-sorta-not-really emulates Greek history. Most of the important city states had colonies as far away as Marseilles. Second, the tribute from these areas explains why Ilion can be as large as I want it to be without straining credibility too much. It's obviously not realistic, but it's realistic enough for me. Third, it gives a kind of Wild West feeling to the setting, which I enjoy. It is basically the same set up as Beyond the Black River, and one cannot go wrong with Robert E. Howard. Finally, and most importantly, it supports adventure.
Now the player characters can be hired by a band of naive settlers seeking stout men to protect them from the horrors of the night. A local noble might want some forlorn peak and the caves about it cleared of the savage beasts and chthonic races that live there so that he can mine it for precious iron. The PCs could flee to a border fort to hide from the authorities after killing a powerful noble. Scouts may spot a long forgotten temple to an unknown and blood thirsty god. Rumors could say treasure lies within...
You get the idea.
Now all that's left to do is draw up a map and sprinkle it with these forts and some ruins and I can continue on from there. More Underworld material to follow.