The POD is that Balthasar Gérard dies before getting to assassinate William the Silent. Result, William doesn't die and get's to live to an cranky old age and the weakening of the stadtholdership which took place after William's death in OTL never happens, derailing the rise of the rich citizen and merchants class to their OTL dominant position. The Netherlands eventually are divided into four Bundes-nations (?). Furthermore, the Netherlands fail to seperate from the Holy Roman Empire after the Peace of Münster.
This is my speculation on what could happen if William the Silent survived. What's your opinion?
I don't know.
For one, William of Orange was already quite old for the time (54), so it's hard to image him living decades more.
Furthermore, the Stadtholdership reached it's apex during the sons of William of Orange, Maurice and Frederick-Henry. The weakening that occured happened because William II tried to capture Amsterdam to intimidate the 'regenten', but failed and died a few months later with only an unborn son (the later William III). His opponents then decided they were better off without a Stadtholder.
As for my ideas concerning no Baltasar Gerardts: IMO William's role in the 1580s was over. He was a political leader, not a gifted general, to put it mildly. The Union of Utrecht needed a Maurice, not a William.
The Duke of Parma was besieging Antwerp at the time of William's death and would rule supreme in the Low Countries until ordered to fight against the Huegenots in France.
Around that time, William would probably die too, giving the opportunity to his son Maurice to shift the tide in the military confrontation like OTL.