This is an idea for my game, ''Van Dread''(title pending). Van Dread is a horror/pulpy game set in the modern age. Characters play former insane asylum inmates that are set free to fight supernatural threats under the guidance of a mysterious organization.
My initial concept for the monsters they fight and the usual story format didn't work for me(Silent Hill-esque monsters, game limited to ghosts) so I'm in need of new antagonists.
I know the game is pulpy and focus a lot on action, with little investigation and horror thrown in, so this is what's important to me:

1. Antagonists should often be multiple, or at least have many goons and pawns. A vampire for example should have lots of vampire underlings, to provide cannon fodder until the big climax.
2. The Big Bad guy should have one or more weaknesses that provide a means to even the odds. They shouldn't be too crippling though.
3. Monsters should be original, or at least a reincorporation of traditional monsters. For example, a Vampire that looks like a human beeing but can grow tentacles and spit acid. That's not a very original example, granted, but I'm taking the easy road.

On to the main issue. My problem is, do I create an original setting and offer a ''monster manual'' of sorts(ala CoC, DG, etc) or do I go the other way and provide the means for a GM or the players to create a completely original monster? Maybe a mix of the two?
Here's what I have:
Classify monsters according to their archetypes: Vampires (feed on humans to survive, parasites), beasts (hunt humans to survice, werewolves and their ilk), elder gods (cthulhu-like entities, etc), cults and the many(zombies, alien invasion type things, etc)
and then provide some well thought out examples as a sample setting. Also, provide rules to create your own monsters, with a point buy system like ORE or even The Guide to the Fomori book by WW. It would work like this:

I want a game with parasites that take over the host and make them into monsters. I want the parasite to be intelligent and feed on humans, so I go with the Vampire archetype, since The Many are mostly about taking over the world or completely mindless.
The vampire needs an origin(means of infection). I come up with a story about intelligent fleas that have a taste for blood. The Big Bad is the original vampire. He is stronger, bigger, more freakish and has weird powers.
I need a purpose for him so I come up with him wanting to take over a small town and later on expand. He uses his minions and more fleas to take over the inhabitants ala Invasion of the Body Snatchers. He butchers a few of them too.
But what kind of powers does this monster and his minions have? I have 10 points to use. I look at the pages for the monster abilities and choose:

Tentacles at 2 points. They reach 4 feet and sprout from my back. They function like extra limbs and they can hit or grab.
Spit Acid at 3 points. It causes x amount of damage. It looks like black paint and causes horrible burns.
Superhuman Strength at 3 points. Adds 3 strength points to the vampire, staying true to vampire mythology.
Escape measure for 2 points. He can abandon the host at will. Useful for sequels.

I like this but I dont know if this would work, especially coming up with your own origin and purpose as well as weakness. Maybe a Burning Empires-style mechanic is needed. But then again I want to keep this simple.

Opinions?

Crossposting this to the Forge and SG.