I recently made a character to play in a Vampire: Dark Ages game (though the superflu may mean that we don't actually end up playing for a while), and it occurred to me that "Dark Ages" would work as a Dresden Files title.
Harry has a habit of skirting or occasionally breaking (with some degree of justification) the White Council's laws of magic, but we don't know of him messing with chronomancy yet. What if Harry did go back in time and end up becoming/training the original Merlin? Maybe Mordred/Kemmler/Cowl are likewise related? Maybe by the end of the series, Harry becomes the Blackstaff and ends up time-hopping to counter & combat the entity that turns out to be more than just an extremely powerful necromancer. Maybe that entity is similar to the Fallen, and killing one host will only delay it for a time until it infects the mind of another mortal. If Harry discovers that black magic acts as a gateway to let this entity into a wizard's mind, maybe he forms the White Council and its laws as a way to proactively detect and hunt budding dark wizards before they can grow powerful enough to become the next embodiment of the capital 'D' Dark. If strict enforcement of those laws had cost his younger self his head, that would be ironic, but also a serious paradox, so maybe he's had to tweak the timestream in other ways to avoid that, which could take literal Ages of work... Dark Ages!
Of course I'm likely nowhere close, but with the excitement of the next book finally coming out, such wild speculations come to mind.