So what would you folks like to encounter when you stumble upon a ruined city out in the wilderness?
One of my favorite troupes has to be the Death-cult residing in ancient ruins preforming horrible rituals for their fiendish deities. Players can choose to charge in with spell and sword ready or kill of a few guards and sneak in wearing their robes. They better not waste time though, or the horrible ritual to summon [insert elder God here] the Eater of Worlds! I'm really thinking somewhere between Col. Kurtz and Temple of Doom here.
Another possible fun encounter would be a Necropolis in some foggy bog. Forgotten by nations and time, a lonely city deep in the marsh toils tirelessly to endure the harsh surroundings as the rotting peasants mend the defenses. Ruled by an arbitrary lich, the lifeless peasants and guards live life as they have for centuries. Raids for fresh 'subjects' are infrequent but bloody. The nearby hamlets of the living speak in hushed tones about the masses of lifeless warriors gathering deep in the marshlands.
Lastly another sort of ruin encounter I would like to see is ruins inhabited by some new colony of another culture. I'm thinking of the surface Drow who dwell in Myth Drannor or the Forsworn of Skyrim in the tombs of the Dragur. Old architecture juxtaposed with another group's order makes for interesting areas. Perhaps the Mad lord of the bandits of the Black-woods declares himself King of lost Carcosa. What if the forgotten inhabitants of the city awaken to find their old homes occupied by strangers?