There seems to be an attitude both amongst the 2008 post and the new ones that contested or exclave cities could become independent city-states. Essentially I'm referring to the Calais and the Berwicks and to a lesser extent the Dublins of this thread. I'm not sure how this works. These cities (possible exception of places like Dublin) were all cities which were wanted by several countries, were fought over repeatedly by them, and crucially though possibly attractive and flourishing mercantile cities, they are places which massively depended on the protection and trade of their owning nation for their survival.
Calais, for example, boomed under English trade. If the English no longer own it, why would they trade through it? And why would the French? Suddenly it loses its only money-maker. Now, what's stopping the French from simply rolling in and annexing the place? The land is technically French, it has no claim to independence, and no-one would so much as blink an eyelid at the move. In fact it would cause decidedly greater stirrings for Calais to declare independence because...well, what possible use has it got for independence? It would be a good way to turn the city into a run-down wasteland, assuming it could actually stay independent. Berwick also. Why would it ever declare independence when it has such an important role to play in Anglo-Scottish relations? Why would it want to be independent when its citizens must surely have found common identity with one nation or the other on their borders. It's a bit like suggesting that, say, Bristol would one day up and decide it didn't want to be English any more. And again, if Berwick ever did declare independence, who can actually see England and Scotland actually allowing it to survive as an independent city-state when it makes far more sense to conquer it and continue using it as a vital military outpost on the border.
Better ideas for potential city-states come in areas of weak authority over long periods of time, in locations of no strategic importance, and where the concept of a nation-state was slow to form. Perhaps a more chaotic Teutonic secularisation could result in an area (say, Livonia) fragmenting, with a few states forming, one of which being an insignificant town somewhere, perhaps on an island. If events can be manipulated to somehow make the reconquest/reunification of that area take a long time, it's a possibility one of them could stay independent. Other possibilities include somewhere like the Pirate Republic of Bou Regreg down the Atlantic coast of Morocco, or possibly other parts of Italy.