If Charles V and (especially) Phillip II had handled the situation differently, things would have been vastly different. Preventing the Dutch Revolt wouldn't actually have been that hard, even. While there were various causes to it, in Charles V had decided to be more tolerant of the local sovereignty of the Seventeen Provinces... that would do it. Critically, he ordered the Estates of each province to write down their local laws and send those law codes to him for approval (and, implicitly, standardisation). More critically, he was very set against protestantism, and while the Low Countries weren't actually causing trouble, his anti-protestant policies actually caused the protestants in the Low Countries to radicalise. Even after he handed over the Seventeen Provinces to Phillip II, the whole uprising and war of independence could still have been prevented if Phillip had gone with a move towards toleration, instead of going all-out with the "hard line"-policy as in OTL.
Either Charles or Phillip choosing an alternative approach is not impossible: both were exceptionally close to William of Orange, who became the leader of the revolting nobles. It's conceivable this close bond could have led to a POD whereby either one cold be swayed at a crucial moment. The ramifications would be enormous. Consider that prior to the war of independence, there was a considerable concentration of Calvinists in the south, centred on Antwerp. They all had to flee north during the conflict. Antwerp fell to the Habsburgs, and the south went all Catholic. At the same time, Catholics had to flee inxcreasingly Calvinist areas, and Catholic churches were demolished (and appriated for Calvinist use) in a terrible bout of iconclasm up north. Very interestingly, the eastern parts of what became the Dutch republic had also had quite a lot of Lutherans, who were later displaced by Calvinists.
So... the Dutch Revolt being avoided would mean: united Seventeen Provinces, under Habsburg rule but with exceptionally great autonomy, and special status regarding religious freedom (basically: the people can do what they want so long as they accept the government is Catholic). Much more Catholicism remaining throughout the north. Much more Calvinism remaining in the south. Lutheranism probably staying predominant in certain parts of the northeast. No iconoclasm: lots of Catholic architecture remaining undamaged, and Calvinists building their own churches instead of heavily damaging and then appropriating Catholic ones.
Even if the tensions eventually do become a problem later on, and the Netherlands still split off from Habsburg rule... if enough time passes before that happens, the above factors would lead the Seventeen Provinces to have gradually become more religiously diverse across the board. Later independence would probably mean that all seventeen provinces secede together. Resulting in a religiously diverse and tolerant ''Super-Netherlands'' (and no independent Belgium).
