So I've been reading my Night Shift and Esoteric Enterprises lately. (To be honest I don't see the appeal of using OSR's level-based mechanics, but whatever.) EE's page on vampires is pretty funny because it treats vampirism as a niche occupied by multiple beings with wildly different origins, which is a first in roleplaying games as far as I know aside from Dresden Files. What's even funnier is that, for comparison, the lycanthropy page just copies D&D-style lycanthropes. This has further solidified my interest in working on system-less "Night Worlds" rather than trying to design my own mechanics.
I've been writing some notes for how to categorize and design Night Worlds. These are by no means complete, but you might find them interesting. What I started out with was a set of parameters or axes:
History/Timeline. Urban fantasy is generally accepted to be able to take place from the 19th through 21st centuries. Less commonly settings take place in the future and may crossover with scifi genres like cyberpunk or space opera or post-apocalypse, while others take place in full-blown alternate history scenarios if the supernatural was already revealed or integrated (see Revelation and Integration below).
Revelation. The degree to which the public is aware of the paranormal is another key parameter. This ranges from the public being completely unawares to the public being fully aware. In the latter case, when the revelation occurred and how that affected history and geopolitics becomes important. If the revelation occurred in the past, then the present may be full-blown alternate history.
Integration. How integrated the supernatural is into history is another parameter, whether secret or not. Is there an invisible history where secret societies have been fighting each other since time immemorial? Or are magical beings recent interlopers from another world, perhaps part of the latest of several previous migrations?
Diversity. The diversity of paranormal phenomena is another key parameter. Some settings may be limited to one type of paranormal phenomenon such as ghosts, vampires, psychics, or whatever, while others may be “kitchen sink” fantasy where every myth is true and has representation somewhere. In more scifi-ish settings, the supernatural may be explained as aliens… or aliens may be just another weird phenomenon alongside magic and demons. Some settings may attempt to explain everything using a grand unified magical theory.
Protagonists. Who are the protagonists of a typical adventure? People who hunt monsters, whether as amateurs or part of a secret society? Paranormal investigators, whether amateur ghost busters or federal-funded agents? Some flavor of magical freak themselves, playing detective, superhero, villain, or soap opera melodrama?
There are any number of other potential parameters but that's as far as I got. The intent is to provide a series of spectrums rather than the more arbitrary categorization of "shadow hunters", "modern conspiracy", "soap opera", and "gonzo" I offered a couple pages back.