Sorry I didn't get to respond with more in depth answers to your questions. I think it's a valid point...how do you keep up the tension of a horror campaign? Here are my thoughts:
First of all, the term "slasher" can apply to a lot of things. In the book, I've got the creepy bugs that look like people, the dream slasher, the demonic slasher, the possessed doll, the weird alien guy with the killer spheres, the pumpkin demon, and more. All of them are very different, and normally would be extremely difficult to put together in one game.
That said, there's a few ways you can link them together. Obviously, they're all horror staples, right? As you can see from the suggestions above, opening up a Hell portal might unleash these things. Okay, maybe not the creepy bugs, but certainly the dream slasher, demonic slasher, possessed doll, and pumpkin demon (assuming you build an adventure around each) could definitely keep things going for awhile. In essence, there is no "one bad guy" but it's like trying to stop a plague. You vaccinate one area, and the plague spreads somewhere else. A demon possesses a doll in one town, but in another someone is so tempted for revenge that they create a pumpkin demon. It's not the slashers themselves that are the meat of the story, but how circumstances come about to cause them to exist.
Mind you, I like my horror wacky and violent, but I'm sensing that you want something more substantial from the game in the long term.
I'd take a cue from Stephen King's writing style (specifically, "On Writing"). What makes his books truly scary is that they aren't just about ghosts and monsters. They're about broken families and shattered lives. So the possessed doll might belong to an abandoned little girl who is ignored by her family. And the pumpkin demon might be summoned by the class nerd who was the victim of a nasty Halloween prank. This means there's more to the adventure than just killing the slasher...it's about finding out why it exists and stopping it at the source.
You'll see this comment about ghosts in the MM too--they shouldn't just be killed in combat, but philosophically defeated/resolved. Otherwise, the bad guy keeps coming back.
So maybe the only way to stop a slasher in one scenario is to get him to realize his mother gave him up for adoption against her will. Or make the vengeful spirit of a moonstruck teen realize that her crush was actually planning to go to the prom with her, but he really DID get in a car accident that night.
The other possibility, one that's a bit cheesy but I like anyway, is that the collective subconscious of reality begins manifesting itself. There was an editorial in the Week, recently an article about the apprehension of little girls
http://www.theweekmagazine.com/article.asp?id=1040
My daughter’s special fear, apparently, is not all that unusual. Children’s clothing designer Lauren Scott last week introduced a new line of pajamas embedded with radio frequency identification tags, in case your kids are carried off in the middle of the night. During the daylight hours, Salon.com reports, you can track them with a GPS Kid Locator Tracker Backpack, or a GPS Personal Locator and Wristwatch. Away on business? Microsoft is selling a teddy bear with webcams in its eyes, so you can monitor your kid’s bedroom via the Internet.
The statistics, of course, would suggest that this is an overreaction: Crime is actually down about 40 percent from the 1980s and ’90s. Abductions by strangers remain extremely rare—about 150 every year. Kids are far more likely to die of the flu or to drown or be killed in car accidents. Even with terrorism and pedophiles on the front pages, the experts insist, our kids are not growing up in a more dangerous world. We only imagine it so.
I’ve tried convincing Jessie of this, but she isn’t buying it, either.
All it would take is just a little bit of the supernatural to start manifesting what people fear most. It's bad enough that beings might manifest in the shadows, fully formed...but what if people became the worst of what others thought of them?
What if, the big scary guy (who's actually a lovable lug) becomes the murderous killer because people think he looks like one? This could give a string of slashers who aren't necessarily your average slashers but became them because...well, because they look the part.
Another possibility is mix the bad guys, using ghosts, zombies, and slashers for different sessions. Slashers are a pretty broad group (you'll see this come up in the disagreement between the NCRPC and PRU over the "goalie slasher"). For a great example of a slasher AND a ghost, see the Frighteners.
I'm not sure if this ramble is really answering your question, but those are the thoughts that came to mind. Does that help at all?
