Mary II's fertility was ruined by an illness contracted while she was pregnant with her first child in 1679. The child presumably miscarried, and although there were rumors of pregnancies until 1683, her inability to carry a child to term seemed like an inverted joke of her sister's Ceres-like fertility. Now, my question is this, say she doesn't miscarry the child, but carries it to term and it is a healthy bouncing baby boy. However, much like her namesake and mother-in-law, William III dies during the pregnancy. (His health was pretty dodgy most of his life AFAIK, with his asthma and whatnot, and his OTL death was rather a fluke in any case) so say his horse stumbles on a mole hill in 1679 or 1680 instead of 1703. Now, once again the Dutch Estates are left with an infant stadtholder, they're just coming off the rampjaar of the (First) Stadtholderless Period, and his mother is a king's (grand)daughter, and it's looking eerily like a deja vu of 1651 again.

So what happens next? Mary get shipped back to England and remarried (where?)? Or does she stay in Holland and fight for her son's rights? And so on and so forth...
Nothing, the position of stadtholder was elective, not hereditary, even if a single family monopolized the position, think about North Korea or Venice for similar examples. They either wouldn't elect anyone, elect a relative of William III, or a new patrician family rises to power.
Well, that might be a possibility, but I'm just thinking of how they went through stadtholderless periods before and after William III, but seemed to have no problem with four different successive regents managing for Willem V.

That said, though, the Netherlands is in a far different place in 1680s than in the 1750s. Would the stadtholderate be removed to the Nassau-Dietz branch? Or would they grudgingly accept little Willem Karel Jakobus as Willem IV?

Back in England, will Charles II not be remembering how the Dutch treated his sister, and try to weigh in on them for his niece? Or, if he sees that this is hopeless, simply recall her to England and start casting around for a new husband for her?