What do you mean we can't do an urban fantasy campaign in DnD? You can do anything fantasy in DnD. There's even some rules for adding sci-fi into DnD. It's a pretty versatile game.
I mean, you
can, but then it's essentially just D20 modern.
Also, on reflection you might not be aware of what 'urban fantasy' connotes, but to clarify - while it might take the form of dungeonpunk (which, I'll note, meshes perfectly with some D&D settings - Planescape or Eberron, for example, are very dungeonpunk), other examples of urban fantasy would be things like
The Wolf Among Us,
The Dresden Files, or
Supernatural. 'Urban fantasy' in its generic mode is very modern, and D&D, regardless of edition, is almost invariably in a high fantasy - and thus by extension medieval - setting.
Obviously there's differences if you switch settings - Dark Sun, Ravenloft, and Planescape all stand out (Dying Earth, Gothic horror, dungeonpunk) - but the default settings D&D has been set in (Forgotten Realms, Greyhawk, and Mystara), are all quintessentially high fantasy places (Golarion, the default setting for Pathfinder, is the same way).
So essentially what I was trying to get at was that if you were trying to have a Buffy the Vampire Slayer experience, your best bet would probably be something more like FATE or a WoD game (Hunter: The Vigil probably) rather than D&D.
I've heard that this system doesn't translate well to forum play because the Fate mechanic often requires some haggling, so this might fall off the list if the RP is 100% forum-based.
Well, as the title of the thread suggests, I was assuming it was going to be Discord-based, where people would pick a time to all be online and in voicechat, and play that way, using Sidekick for dicerolls.
As a personal note, I find FATE to be fantastic,
especially if the game shifts towards troupe-based play instead of being focused on a single party of PCs,
and it's incredibly flexible; I've run FATE games in high fantasy (Glorantha; I didn't want to learn Runequest but I'd played KODP and loved it), hard-SF (some may recall my Diaspora RP proposal, which I wound up using in my RPG group so that scratched the itch for that and that's why I never went back to it, I'm a bad person), and superhero settings, and it's worked great in all cases.
It
does lend itself much more to seat-of-the-pants sort of games, rather than intricately plotted ones, but the flipside is that it also often requires a bit more out of players since they can often spend fate points to change certain elements in gameplay (e.g. "I need a Computer roll if you want to try and figure out his password". 'What if I spend a fate point, could we say that the director is forgetful, and so he left his computer password on a post-it note?' "I like that idea, I'll add that to him as an aspect, Absent-Minded, no need for that roll.") If that seems like it could make things too easy, the flipside is that people tend to hoard their fate points in case they'll need them for something later so the notion that stuff like that may become gamebreaking sort of flies out the window pretty quick.