The thing about STALKER is not so much the post-apoc setting or the mercenaries or the factions and politics, or even the monsters, but two things - the safe havens and trading places, bars etc; and the simple weirdness of the anomalies and the weird radiation effects. Doing something with the anomalies so that there is some kind of strange puzzle to unravel as a party would be awesome - I don't just mean artifact collecting but something almost crazy and interdimensional. With the anomalies you could take a STALKER-based story almost anywhere, literally.
I also personally found the city settings much creepier than the open countryside. One thing that the STALKER franchise did exceptionally well was the settings, the colours, the visuals and the sound effects. The gritiness. The plain scaryness. capturing some of that in a group RP would be so cool.