Similarly, the usefulness of an invisible agent hinges on no potential enemy knowing that an invisible agent exists, because they'll start retraining their guards and investing in thermal goggles. So here's the scenario. The player is a disgraced soldier who volunteered for the weapons research division of an ethically dubious PMC (pardon the tautology there). He is turned permanently invisible by an extremely small division within the company that's almost universally unheard of and staffed by as small a number of individuals as possible to preserve secrecy. Your services are offered to the PMC's clients as undercover work with no detail given besides a guarantee of success.
It'd be mission-based and each mission would hinge on you not letting bystanders realize there's someone in the room with them. Maybe each location has a "suspicion" meter, which goes up when you draw significant attention to yourself. So you can't touch anyone or let them see you pawing through documents or carrying keycards, but even just sneaking up behind people is harder than it sounds. I don't know if you've ever tried to move around unnoticed in a small, silent room containing another person that you intend to murder - I mean, give a hilarious surprise to - but it's a lot harder than you think and there are far more factors than simply visual ones. Have you ever tried going up or down a flight of stairs completely silently? Even if you did manage it, being silent still gives people this weird sense that someone close by is trying to be conspicuously silent. Your best bet would be to distract individuals with manufactured background noise and thrown objects while chucking significant objects out windows and down garbage chutes to be collected later before anyone can see you carting them about.
There could even be a mechanic where you reduce the mission's suspicion level by making characters believe that suspicious characters are going mad. In fact, the first mission would probably just be a tutorial in this, in which you discredit the claims of some mouthy official by visiting him when he's alone at home and throwing his ornaments around. If only one NPC sees you open a door or knock a cup off a desk then they'd set off an alert and other people would take a look around for them, but once the alert stands down if you kept doing it then security would get more and more impatient with the guy until you can start juggling plates right in front of them and no-one cares, because of the "boy who cried wolf" factor. But be careful! If just one other NPC sees your antics at the same time as your target, then your target will suddenly regain their credibility and undo all your hard work. Still, no reason two people can't go doo-lally on the same shift, right?
What I think would make this game interesting is that the player would have to learn a whole new set of rules. You'd be immediately noticed if you ventured into water, and tracking through a dirty environment might leave you with compromising stains, or make you leave noticeable footprints. You'd have to have your wits about you at all times in case a crowd of gossiping scientists are bearing down the hallway towards you and you can't just knock guards out willy-nilly unless you can make it look like a particularly violent spasm.
I can think of at least one reason why publishers wouldn't go for this, though: because the main character is invisible, you can't include a five-cent action figure of him with the "special edition" and charge an extra thirty bucks for it. Or perhaps you could. Just throw in an empty plinth and hope it wins points with the irony crowd.
Yahtzee is a British-born, currently Australian-based writer and gamer with a sweet hat and a chip on his shoulder. When he isn't talking very fast into a headset mic he also designs freeware adventure games and writes the back page column for PC Gamer, who are too important to mention us. His personal site is www.fullyramblomatic.com.