Ignoring the myriad microstates which actually did exist, how about a breakdown of French control over Aquitaine around the 1100-1400s? Or heck, a total breakdown of control from Paris, there were numerous conspiracies and intrigues where powerful French nobles tried to become quasi-independent and force the Kings of France to recognise their right to do what they wanted. Aquitaine, though, barely even had any Ducal control, let alone royal. It was very hard for the Dukes of Aquitaine to make their control stretch any further than literally a whole three cities and a tiny bit of hinterland. If you remove the overlordship of Paris, the French nobles will probably descend into an Italy-like mix of states of varying sizes and ambitions, while Aquitaine, if you can prevent someone walking in and overrunning the place, could become the tiny, western European equivalent of the Chinese warlord states. Literally, it was a mash of vassals who ignored the Duke, and essentially spent most of their time making alliances, starting fief v fief wars over blood feuds, and etc. It'd be fantastic if they survived.
It's a real push, but Frisia would be good, too. Have someone with little interest in centralisation become the Duke of Holland around the 1400s (marriage to Jacqueline of Hainaut works) and they inherit overlordship of Frisia, which in this era stretched along the North Sea coast almost to Denmark. If they can have the HRE recognise their right to rule there (probably by virtue of in turn, strengthening their own vassalage to the HRE) and then lead a campaign to have Frisia recognise them too, there's a wealth of possibilities. Frisia in the middle ages and the mediaeval era was a mix of town and village communities, hundreds of them, where virtually not a single settlement recognised any more than a loose connection with the local town as protector. Have them get an overlord who doesn't care about them, so they can be safe from invasion, and that whole area could slowly turn into a mass of independently governed townships, each of which would probably claim stateship in their own right. Kind of like a Holy Roman Empire-lite, I guess. You could even end up with a HRE-in-a-HRE.
Or how about the
Republic of Bou Regreg, a pirate republic which extended over a mere two cities, two cities which were conjoined at a river mouth anyway?