Use it as a seed - the idea of a wizard in a modern city. Play with the idea: what about a divine-magic-wielding priest? A scientist who discovers magic in subatomic physics? An alternate history where Benjamin Franklin discovered not electricity but a magical "element" à la the Greek elements (air, earth, fire, water)?
Thing about Franklin is, he's one of these real people who does too much and achieves too much not to come across as a Mary Sue in fiction.
The thing about most urban fantasy, including the Dresden Files, is that to take place in a recognisable modern world there has to be some reason or another for the supernatural stuff to be largely hidden or unknown. Which I can accept as a genre convention for the sake of enjoying the Dresden Files and books in that genre, but is not a thing that interests me much as a writer. One of the things I have on the backburner (about 60,000 words done, but other projects foregrounded for the next couple of months) is a world where magic was formalised around the time of Newton, went through a paradigm shift around the turn of the twentieth century which made it more accessible, and has
drastically changed things since; there have been four World Wars, and there's a White Russian remnant state on the Moon, where my protagonist will be emigrating in the next section.
Find something you're not sure about, change it, and see what follows.
