It seems to me the Netherlands were always a bit a special case.
After all, the greater Netherlands were Frisia plus old west-Francia, where Germany was east Francia with a lot of Bavarians mixed in. In addition, it had been originally assigned to Lotharingia rather than East-Francia, and ended up split between France and Germany. To a large extent, the Netherlands are 'that part of Germany where the Flemish mattered' (the Dutch Republic having strong influence from Flemish refugees, too, but it started before). Now, of course the same thing (cross-border influence) happened all over Europe and Germany, with Bavaria/Austria far more concerned with the Magyars, and the whole Prussia thing in the east, nevermind cases like Schleswig-Holstein where the area seemed to act more like part of Scandinavia.
In that sense, I'd say the 100YW was the essential trigger for the Dutch identity, by creating Flanders as a semi-independent entity (along with most French regions), and the Burgundian inheritances cemented the rise of the Netherlands as a separate entity by insulating Flanders from the subsequent re-centralization of France. Of course Flanders and the Dutch identity were already beginning to arise before, mostly in the Flanders-Brabant-Holland-Utrecht area (Gelre being usually far more involved with the Rhenish part of the HRE, which in the end was split between Dutch and German). But it was all a border zone, hence why Brabantine and Jülichian troops fought on the Flemish side in the battle of the golden spurs.
The borders of this area were still very fuzzy; Paris was absolutely not part of it, but anything north of the Ile de France was almost open game (Picardy never really joined, but Hainaut and Arras were at times as part of the Dutch identity as Utrecht or Amsterdam), and east the Frisian, Rhenish & Westphalian regions were on the wings (the Jülich/Berg/Cleves duchies provided half the brides for Dutch noblemen, and vice versa).
So if you want a recognizably Dutch identity for as large an area as possible, you'd probably be looking at a ring of Bremen, Brunswick, Nassau, Luxemburg, Rethel/Valois, Picardie as the first regions outside the country. You could probably include one of those border regions without compromising the fundamentally Dutch character of this state, but more and it devolves into either a more Saxon, German or French identity.