I really like The Thing idea. Some problems (not insurmountable) which I see with it are mentioned below. Here's what I see as an approach.
A remote Artic station is an excellent place for the research phase of the adventure. It's creepy, cold, and eerily quite.
Clues found at the scene would point the party back to a densely populated area. The knowledge of how quickly our Thing could spread in an urban environment should be more than enough to make our players shudder.
It's easy to envision a chase scene in the bitter cold of the Artic, or a battle with henchmen en route to the final local.
Now the showdown with the true villain would be tricky to pull off, especially if The Thing is one of the players!
Okay, so we have a basic outline. Research occurs at arctic station (Ice Station Zebra?). Why are the PCs there? Maybe they are sent to find out why the station is no longer communicating. Maybe they work there. To borrow from the movie -- PCs are American (or whatever) base members and are asked by the Norwegian government to travel to nearby Norwegian station to find out why comms are out. Find mayhem, maybe even a couple of survivors who act crazy (shooting at the players and yelling in Norwegian). Research ensues. Getting computers up and running, cracking codes. Going through paperwork. "Anyone read Norwegian?" Good search results turn up blurry photographs hinting at dark discoveries. Keep players on their toes with discoveries that could both be clues or misleading (why did one of the dead Norwegians have a swastika tatoo? why does one of them have books on lost Nazi submarines? why was one of them reading the baghavad gita in the original sanskrit? why does one of them have a note in his journal, dated 2 days prior: "The American knows something.")
How do you keep the tension, however, of wondering if any of them are infected? I suggest handing out a slip of paper to each early on -- for each of them it says: "Nothing of interest." Once they discover more about what is going on (virus, aliens, whatever), hand out another piece of paper to each which says: "You are not infected." or "You are a normal human being." or maybe more mysterious: "You are who you think you are."
But here is a larger problem. Unlike in the movies, once a group of players figures out what is going on, the PCs will stick together. How can one of them sabotage the radio, commit murder, eat the sled dogs, etc. if they are sticking together and if they themselves do not even know that they are the Thing? Here's a problem. I think the obvious solution would be to give the PCs form the beginning different agendas and let them know players will receive points and be ranked for achieving certain goals during the session, with a "winner" announced at the end. Maybe hand these out at the beginning somewhat randomly.
- Your PC has a disease he has been hiding (something contagious and with a stigma, let them know keeping the disease hidden is a primary goal for the session) and this would be revealed by a blood test. (Not good science, I know, but stay with me...)
- Your PC is a government agent. You have been sent to the arctic station to investigate UFO sightings and must immediately report back upon discovering information while keeping your identity a secret.
- Your PC has reason to believe a Nazi submarine full of gold arrived at a secreat arctic base in March 1945 and is on the trail to finding it. Your PC has no intention of sharing the gold with anyone else. Worse, the PCs quarters at the base contains barely hidden documents on very similar to those discovered in the quarters of one of the Norwegians. Other PCs will suspect something if yours are discovered...
- Your PC cares only for his own survival. Escaping with his life is his primary goal and once dangers appear, he should make every effort to escape, leaving the others to their dooms.
- and so on, as necessary
This kind of thing requires a lot of handing notes back and forth, though and may be too time consuming for a single 4 hours session.
Thinking out loud here.