What if the Dutch, at that time under the Union of Brussels that spanned both the southern and northern Netherlands, had managed to defeat the Spanish forces hassling them at the Battle of Gembloux, in 1578?
IOTL, the battle had significant consequences for the history of the Low Countries -- after this Spanish victory, the provinces on the border with France (Wallonia) signed the Union of Arras and declared for the Habsburg Spanish king Philip II. As a response, the Protestant rebels of the northern provinces signed the Union of Utrecht, which would eventually develop into the Dutch Republic.
Had the Dutch managed to defeat the Spanish in such a decisive battle, could the Union of Brussels be preserved? Could William the Silent become its head of state if he lives longer?
Oh, and one more very important question: Alexander Farnese, Duke of Parma, was personally present at the battle. If the Dutch manage to kill him, he would be succeeded by his very young son Ranuccio as Duke of Parma. Considering that Ranuccio would be tutored by a regency and that he was dynastically very close to the Portuguese throne, how does this affect the Portuguese Succession Crisis, assuming that Sebastian still dies in Morocco?