So, with larger territory, and more Provinces, how would this effect politics in this larger Dutch Republic?
That would be very interesting, especialy if Flanders and Brabant remains protestant. Also keep in mind, Flandersand Brabant did join the Union of Utrecht and sign the act of abjuration, it was simply that they were reconquered by the Spanish. And that last part is the big problem with the POD. At the start of the Dutch revolt the Spanish were effectively winning the revolt. They recaptured Flanders and Brabant pretty quickly (and this was in many ways the core area of the revolt. Antwerp was effectively the capital of the Dutch revolt) and only when the Spanish reached Holland, the tide was turned, only it was too late to recapture Flanders and Brabant (well most of Flanders and half of Brabant). If you want all 17 provinces to become part of the Dutch republic, you need a very early POD in which the Spanish screw up very quickly and very badly. I would say that there are several POD that would move the Dutch border south, but I think it is extremely hard to move the Dutch border so far south the Artois, Namur and Hainaut. Especialy since Artois and Hainaut signed the union of Arras* in which they announced their support for Spain. Simply put the Walloon provinces did not support the Dutch revolt, while the Dutch (and Flemish) provinces did. So if you want all 17 provinces to be included, not only do you need to avoid the Spanish capturing large parts of the Union of Utrecht, you need the Dutch to be able to capture provinces that not actualy supported the revolt. That is going to be very hard.
If you want to know what kind of consequences it would have, it realy depends on when this happens. If Flanders and Brabant remain part of the Netherlands early enough, or are recaptured early enough. They would be treated the same as the other provinces were. OTL Holland was able to dominate the Dutch Republic. In this timeline Holland would not be able to, since Flanders and Brabant would be as important, or probably even more important than Holland. If Brabant and Flanders are recaptured late in the war, both probably end up as generality lands, like OTL Dutch Brabant did and they would not have the influence or autonomy the provinces had.
Actualy, I would even go so far that with an early POD the entire Dutch republic would look entirely different. It was only during the Dutch revolt (and just after it) it was decided how the Netherlands would take shape politicaly. With an early POD especialy one with such big consequences the entire political system could and would be different. Although there would be some things that would be the same. A very large influence for the cities and compared to other European countries less influence for the nobility and large autonomy for the provinces.
Also, as you noticed I am only talking about Flanders and Brabant. How About Artois, Hainaut, Luxemburg, Namur, etc. That is going to be tough. With an early POD I suspect they would be ruled by a protestant minority (and there were plenty of protestants in those provinces, just not a majority) and only those protestant would have influential positions. That alone would be an interesting (although volatile) situation
*I always liked the Dutch name, unie van Atrecht, since it opposes nicely the unie van Utrecht
