So I decided I'm not going to make a custom system for the game, as that would take too long and could be prone to bugs.
Instead, I am going to use the system present in the Sine Nomine games like Stars Without Number, Other Dust and Spears of the Dawn. There will still be three classes, but I know the system works well so it'll rock.
I'm also changing around how the races work a bit. Instead of being a pure zombie, mummy or whatever, you are instead a human instilled with basically a monstrous bloodline. You'd be human, but you'd have the lycanthrope bloodline. In total, there are 12 bloodlines and then a 13th "humanity" bloodline. Everyone has the humanity bloodline and then a monster bloodline.
The game will then have a corruption scale. If you become more monstrous, you get cool monster powers but also get the drawbacks of being that monster. For example, the lycanthrope bloodline will start off with the weakness of being very violent, unstable and, well, horny, on a certain phase of the moon. At the second level, the lycanthrope will get large amounts of hair and will look strange, but he'll be able to speak to animals now. At the third level, the lycanthrope will be able to shift parts of his body, like turning his hands into claws, but he will become severally allergic to silver and wolf's bane.
At the fourth level of corruption, you have fully turned, and you become an NPC probably. However, if you do awesome humanistic stuff, you'll go down the path of humanity and gain some powers. But as you become more human, you lose all that cool Hammer horror powers.
I'll probably also include rules to basically "multiclass" bloodlines, for those people who want to play a vampire/werewolf or whatever.
The bloodlines so far are Lycanthrope, Vampire, Golem, Hyde, Spirit (angels, demons, etc.), Ghost, Witch, Fae, Mummy, Gill-man, Slasher (serial killer), Zombie and Humanity. I don't know if they will change, but I like the solid power balance of "classic" monsters.