Idea: Dresden Files/Star Trek crossover. Kirk and crew run into their counterparts in a universe where rather than closing off the Nevernever and ending magic on their world forever, humans attain warp through magic.
As the world sat back trying to recover from the devastation of the Third World War, and society scurried about trying to figure out how to rebuild itself in the wake of the destruction that had been caused, the first of a new, or rather old generation of computers began to appear on the electronics market that had all but been destroyed by a number of electromagnetic pulses despite generations of warnings that such things would happen if precautions weren't taken. This new brand of computer that was the result of a couple of generations of tinkering in garages, basements, home laboratories and other such places was powered by something that couldn't be taken out by a mere electromagnetic pulse, something that was as old as the universe, and far older than science. When Arthur C. Clarke had said that any sufficiently advanced technology was indistinguishable from magic, this wasn't exactly what he had in mind. Niven - who had given the converse of Clarke's law - however would probably be laughing his ass off were he still alive.
Zefram Cochrane - who always had trouble with devices more complicated than a toothbrush that weren't EMP hardened - promptly purchased one of these new computers as a backup. If he found it rather similar to the one his maternal grandfather had helped a man named Dresden build as a theoretical project, and still used until his death at the hands of a mugger two decades earlier, he never said a word. When he found this new computer easier to use than the damn glitchy things he'd gotten his engineering degrees on after he'd dropped the whole "Magic" business at the end of his teens and metaphorically turned in his wand, it became his main computer.
The day that really changed the world, and therefore the fate of the galaxy at large had been when a chunk of space debris had dropped from orbit and demolished a small part of the camp, injuring two of the scientists. It had been a stressful week in general as the former main computer had decided to commit suicide a couple days earlier, the jukebox had died the night before (while he refused to admit it, the deaths of the computer and the jukebox were the result of the close proximity of a certain stressed out scientist), and the bar was beginning to run low on alcohol, even the rotgut stuff that was made in the still out back. If that wasn't enough, a set of calculations Zefram had been trying to make refused to add up.
"That's it!" Cochrane yelled as he threw his hands into the air in defeat. "I'm calling in the wizards."
Two weeks later, the project was better than back on track and the scientists had learned to ignore little things like people wandering around carrying staffs and other arcane objects, tiny flying people with oddly colored hair who acted as gofers when they weren't messing around, levitating objects, circular crystal arrays, metal rings, dirt rings, glowing rings of light that didn't come from any apparent source, a skull reading some rather raunchy romance novels, and runes being drawn into the hull of the Phoenix.
Two days after the completion of Cochrane's project, the crew of a Vulcan science vessel landed on Earth to see exactly what it had been that had streaked by at low warp and screwed with their equipment. When they had landed on a newly warp capable world in which the doorway to the Other had not been closed, and discovered that the reason that they had achieved warp had been because the doorway as it were had not been closed, they had been understandably shocked, so shocked in fact that the captain had nearly fallen backwards and landed on his ass.
Leave it to the humans to be the first and only known race in the galaxy to attain warp speed by using magic.
Over the next two centuries, Warp Magic was honed and refined until it was - as Niven would probably put it - almost indistinguishable from technology. Over those two centuries, Starfleet - the exploratory arm of the United Federation of Planets which consisted of a number of races who thought that the humans were a bunch of oddballs but liked their style, and the Vulcans - was formed with the heavy influence of the White Council. Along with the Prime Directive, all members of Starfleet had to swear an oath to uphold the Seven Laws of Magic. While punishments for breaking those laws weren't quite as draconian as they had been in the past, all magic users from the lowliest hedge mage to the mightiest wizard were still bound to these laws.
It is in this universe where magic is - while not nearly as common as science - popularly accepted that the USS Enterprise met its counterpart...
Captain Kirk blinked to clear his vision. The bright light that had suddenly engulfed the ship had nearly blinded him before the screen had blacked it out. When the dancing spots finally vanished, the captain saw that they had not moved, but a ship of unknown manufacture had appeared out of nowhere.
The ship bore, if anything, a striking resemblance to the saucer section of their own vessel. There was however, no signs of a warp propulsion system as the other vessel had neither a lower section nor connections for nacelles. Along with the name of the vessel, which oddly enough was the WCSS Enterprise - unbeknown to the crew of the USS Enterprise, this was the first of the White Court ships which were designed in part by Thomas Raith, head of the reformed White Court and sold to Starfleet in exchange for a couple of colonizable worlds per ship -