The closest thing you had of a metropolitain area in early modern and modern Northern France was Touraine, and by that I mean a relatively coherent network of cities, royal residence and countryside.
Now, if you meant one city toppling or equalling Paris demographically : Paris was a medieval demographic monster, one of the chief cities of Christiendom, being rivaled only by major Italian cities. We're talking at least 50,000 at the beggining of the XIIth century; at least 300,000 at its medieval apogee (conversly, at best 500,000 at its medieval apogee).
Its demographic and political gravity alone made Capetians, originally more centered around Orléans or wandering in Ile-De-France, more and more settling in the city. In a sense, it's less Capetians that made Paris a capital, than Paris making Capetians choosing it as a capital.
It's going to be hard to re-edit somewhere without really tweaking with western Europe history, let alone having a roughly equivalent XVIIth century. It's going to involve a really early PoD, Xth century at very best, and even with a Late Carolingian survival, with Paris remaining a Robertian holding and Laon the Carolingian center...
I don't think Laon, for instance, could come to the same importance Paris had IOTL : I could see a TL where Paris' importance and growth are really limited, being more on par with the German or secondary Italian cities, but I don't think it would imply another french city benefiting from the change.
You'd probably need no-Capetian takeover and more successful Late Carolingians, and maybe a less powerful kingdom of France politically-wise to have a shot at this : it could go either way, different capital or meta-capital made of a network of cities. But if it's not impossible or even implausible in the strictest sense, it's not going to be easy : at least since Carolingian times, Paris was seen as the natural head city of Francia proper.