So basically what you're saying is that your ideal slasher game is, as near as I can tell, pretty much IDENTICAL to the core gameplay of Hitman (except, uhm, you're killing teens and are forced to get "Silent Assassin" on every level), a game that came out in 2000 and has had like 5 sequels? Why not just say I want a slightly tweaked "Hitman" game, but with teen targets?
"Obviously the main objective is to sneak through Chad's party house and murder the targeted teen, but there are going to be a lot of other horny teens in Chad's party house, and if any of them discover your presence and get away from the mission area long enough to call the police then you'll have failed. So naturally you'll want to lure away and kill all the other teens surrounding your target before any of them can realize something's up, almost as if it's a standard slasher movie or something.
It'd be a score based game, and you'd earn points by killing your victims in the most creative ways - waiting for them to stand next to the garbage disposal or the side of a speeding train, you know the drill - and by killing them at the right dramatic moment. "