The original idea was to marry Elizabeth of to the prince of Orange in order to save her older sister, Mary, for a suitably "royal" match. However, needs must, and Mary was married off to William II of Orange at a tender age, never to see her father again.
Now what if (for some reason) the original betrothal had been kept, and Elizabeth had gone to the Dutch, leaving Mary unmarried? What might've happened to Mary? Where would she marry? She'd be too old (maybe) for Louis XIV, but not for certain German princes (amongst whom were counted the future king of Sweden). How might Elizabeth fare in the Netherlands instead? Would her low-church sympathies suit her better to the Dutch temperament? Would she still die so tragically young?