Now, we already have a whole line for ghosts, a Witch (Mismagius), Mummy (Cofagrigus), some Cryptids (Electivire, Abomasnow), technically a Frankenstein's Monster (Mewtwo) and a Vampire Bat (Zubat line), aliens (Elgyem line) but I want more.
Maybe another bat that gives off more of the classical vampire vibe. Or perhaps a Wolf pokemon with a split evolution, one of them being a full blown wolf and the other being a Ghost/Fighting Werewolf. Perhaps a skeletal creature of some sort. A Chupacabra that preys on goat pokemon. A Creature from the black lagoon style mermonster, a pokemon with chainsaw arms that looks like it's wearing a hockey mask (though Excadrill can totally fill in), a pokemon based on Dr. Jekyll and having an ability that lets it schizo between two forms, the zombies from the Red Mist movies in Pokestar studios being real pokemon, and finally some lovecraft stuff.
Well I'm not saying we should get them all at once. But I don't care at all about "overused to death", the biggest reason I want a pokemon based on these things- and a lot of other things too, is because even if it's overused to death a Pokemon based on whatever can still end up being really cool.