In In Harm's Way: StarCluster, one of the possible results in the random world generation is "Normally uninhabitable, with a single large habitable feature". This came up as a result in our current game. This single feature had to fit a Tech Level 10 culture (the highest) with 100s of millions of people. Now, one of the glories of this kind of random generation is forcing you to think outside of the box. It's how I came up with Chariot in the original StarCluster setting.
Chariot, the very first world in the very first system I rolled up, has an average temperature of over 90 degrees F, a poisonous atmosphere, a population of over a billion, and a Tech Level of 5 - steam power. How could a huge culture with steam as it's typical power source - basically a Victorian culture - live on a planet with a poisonous atmosphere and an immense amount of solar radiation? I almost trashed the system I had come up with, but then I thought about it and left it as I rolled it.
That planet, Chariot, eventually turned into one of my favorite settings - a world where people lived on the tops of mountains and never went into the valley, because the poison in the atmosphere was argon, normally inert, but when the pressure went over 3 bars, it became toxic. So the atmosphere was dense - at the lowest possible human habitable areas, it was three times the pressure at sea level on earth. Argon is generally a product of radioactive decay, so a world with high amounts of argon would have high amounts of uranium. So much uranium that it would be sometimes found in natural reactors, moderated by ground water, and usable in it's raw, rich, ore form to boil water to make... steam! Aha! That also increased the chances of mutation, already high, so mutation would have to be a major factor... the chain of reasoning went on and on, culminating an a truly glorious world to adventure in.
So, back to the current dilemma. I asked for ideas around the table. The most popular idea was for a desert world, bare rock and sand. Maybe there would be life at the poles? But the players didn't like that idea - the screwy day-night cycle of a polar civilization didn't appeal to them. Besides, there would be two poles, not a single feature. Then one player suggested a vast crater, shielded from the sand and winds. But how would it be habitable? An asteroid hitting this planet would just make a big dry hole! Another player said "What about a comet? Comets are mostly ice, and high rim walls would tend to keep most of the water inside the crater."
Perfect! So now we are adventuring in a huge crater - so big you can't see across it. There is a high rim which precipitates most water out into the crater. In the center is a big massif where the land rebounded after the strike. Around that are huge lakes, feeding the cloud/rain cycle which dumps water on the rim walls. The lakes would give life to the crater. Huge cities are located here, and vast farmlands, yet outside, life would be precarious at best, a few clumps of green on the other side of the rim wall, trickling out into the stony desert. Awesome!
Here's a map of the habitable crater on Basher: