After much dithering, I'm currently calling it "Dark", largely because first-person shooters of that era tended to have monosyllabic names that didn't really have much to do with their actual content.
Anyway, the PCs' employers would naturally be the weapons R&D branch of some giant megacorporation (not unlike Doom's UAC) who have hired them to quietly take care of some monstrous fuckup in one of their research facilities. The weapons research angle is important because--in accordance with both first-person shooter and survival horror tropes--the characters must have the opportunity to pick up bigger and better guns in the field than the ones they came in with. The employers' monster-ridden facility naturally has to be a source of both mundane and highly experimental weapons and armor.
It also has to be so remote as to be virtually inaccessible. It might be deep underground, underwater, in space, or in another dimension, but for one reason or another, no reinforcements are immediately available, and whatever monsters the corporation has released are (temporarily) contained. The PCs would have only one obvious means of escaping the facility after completing their mission, and that would be whatever conveyance brought them there. For drama's sake, of course, they might lose that option and then have to find another one.
The setting idea I'm really liking, at the moment, would be some space station or or ship that has fallen halfway into another plane of existence through some tear in reality that may or may not have been a naturally-occurring phenomenon. The different physical laws of the other universe--and their applications in weapons development, of course--were the primary subject of the facility's research, but Something Went Wrong, and now the whole thing is sinking inexorably into a dimension inimical to human life.
It also might be fun to have the bulk of the facility be made up of modular cubical sections which have perhaps been rearranged somewhat randomly by a confused or maddened central computer. I dig the idea of actually rolling on a few tables to determine what's behind the next door. (Seriously, in a lot of ways, Rogue was the first survival horror game, largely due to its randomness: You could never count on the game's levels to be even remotely fair to you and your dwindling hit points.)
I'm not too sure exactly what kind of swarming, shootable monsters infest the facility, yet. Certainly, a major component of the hordes must be mutated / possessed / infested / mind-controlled humans; the "former human" is an important genre cornerstone. They're a great source of weapons and ammo, and just about everything in the players' arsenal works on them. Beyond gun-wielding zombies, though? I'd like to Quake's fleshy, quasi-Lovecraftian route, but I've got no final ideas about just what the monsters should be like or where they should come from. Maybe I'll draw on CthulhuTech a bit.
There are a few different possible objectives for the PCs' mission. They could be sent in to the facility of its infestation or simply plant a nuke and leave. They might also have to retrieve data or valuable prototypes, or even specimins of the monsters themselves. And I think I'd actually break with genre conventions and make it a rescue mission, we well: For a change, there'd actually be some non-monsterized survivors to save. These NPCs could also provide aid to the PCs, and both exposition and roleplaying opportunities to the players. Not to mention the fun moral issues they might introduce: Do we save the prototype, or the wounded researcher who might be harboring an interdimensional plague? Should we set the self-destruct timer and get out of here, even though we haven't checked every part of the facility for survivors? Are the guys who shot at us possessed by alien parasites, or just scared?
So, what system would I use for all this? As far as I'm aware, there is no perfect fit for something like this. There are cases to be made for GURPS, d20 Modern, and 3:16 - Carnage Amongst the Stars, and of course I really should consider CthulhuTech's own Framewerk system, but--if only for the purposes of this academic exercise--I want to try hacking my old favorite, Mutants & Masterminds, for the job.
On the face of things, M&M is a terrible fit: Its default power levels are high, most of its crunch is devoted to superhero powers, equipment is regarded as part of a character, ammunition--on the rare occasions that guns are addressed--is assumed to be infinite, etc. I'd get around all of this, though, by just using M&M as a construction kit to develop a much slimmer, simplified player's document, which would largely feature modified lists of skills and feats, and a big equipment section. And, because not all players have my burning love for point-based character generation, I'd offer mix-and-match ability, skill, and equipment packages. Equipment--as is appropriate to the survival horror tone--would be expended, destroyed, and replaced with fresh loot constantly (with no regard to M&M character point totals whatsoever), so each piece of gear would have its own card, including both stats and ammo boxes to check off (and maybe an illustration stolen from a computer game).
I've been doing a lot of thinking on what this hybrid mutant system would look like (oh, man, that is a perfect names for it: "Mutant"), but I'll save the big crunch dump for another post.