Time: Either 2 hours or 3x3hours (I'm really in favour of 2 hours because it makes the experience more intense).
Ingredients: team, law, steel
Premise: The players are selcted to perform a mission "outside" in an environment that casues them to slowly disintegrate.
Characters: The characters allocate stats but most importantly they have to describe something they are leaving behind (maybe this has a game effect). The GM can use this to tug at their heart strings. eg. a family a mother etc. They have to have a tug, a reason to survive.
Mechanics: No matter what precautions they take their bodies will slowly break down. Every 10 minutes (real time) their stat scores will fall until they are dead. These same stats are used to resolve actions. Limbs will begin to fail and they may need to be carried. Of course if they are carried it will cost somebody to carry them.
A tree trunk or some obtacle a player moved easily at the start will be impossible to move unless they work together at the end. At the start though it would cost 1 person "3 points" to move or 4 people 1 point each, so although it is better (for the team) for 1 person to move it that person will take a big hit. They don't recover either, such is the environment that strength used does not even slowly return.
The GM: The GM will have to lay it on thick in some spots for impact.
The GM should spend a few minutes at the start describing a teary fare-well. They also have radio contact throughout and as they die off you can do melodramatic farewells to their children etc. as their companions disappear into the ditance.
The GM will need to describe their physical deteriotration coughing up blood arms and legs giving out.
End: Anyone that makes it back will be so physically wrecked that there is no way they could ever continue as a character in another game.
Why will it be interesting? (Maybe it won't but let's pretend for a minute it will.)
Players will need to sacrifice themselves at certain points as time is passing. Standing (sitting) round debating is killing them all.
Ultimately players will realise they have to work as a team or nobody will make it back.
It's replayable as the first time maybe no-body will mke it back. The next time maybe one person will, the next time maybe 2. The ultimate goal is for the players to work together so well they all make it back.
Problems: PLease tell me... (other than I've not incorporated other two ingredients yet)