Wiwill himself later on wanted to marry countess Mathilde of Limburg-Stirum. The government even got his request to put this to a vote in the Estates-General but decided not to alienate the King by such a move.
It has been rumoured that Willem III had know the countess' mother too well and could not be sure who her biological father was. If not him possibly someone other than the girl's legal father.
He also objected to his son marrying the daughter of one of his subjects.
However both he and Queen Sophie were willing to accept a foreign noble lady at some point.
As dysfunctional as the Royal Couple may have been when it came to the marriage of the firstborn Willem and Sophie seem to have been very much on the same page. Willem was well aware of his own reputation so left the contacts with foreign courts about a potential match of his heir to the Queen.
I think Louise of GB or Alexandra of Denmark would have been an ideal bride for Wiwill. His father would have been happy with either of them. Both pretty and Royal but his wife would have preferred the English candidate simply because she herself was an Anglophile and Alexandra of Denmark was pretty but not an intellectual or artist. Sophie herself was an intellectual and would have gotten on less well with a more conventional daughter-in-law.
their youngest son Alexander made inquiries for Thyra of Denmark and proposed to Frederica of Hanover. The Portugese Infanta Maria Anna (later wife of Guillaume IV of Luxembourg) seems to have been a less likely candidate.
Part of me feels his stepmother Emma of Waldeck-Pyrmont might have been a good wife for him. He might have been an easier husband for her than his father.
At some point the dysfunctional Dutch royal couple had to see that, after their heir reached a certain age, no royal family was overly keen on him - even those who may have coveted a crown for their daughter.
Thankfully, whatever was whispered or not whispered about Thyra back then, a good, solid royal husband was found for her. Her prince-without-a-country may have been seen a second-tier royal, but he proved to be a very satisfactory addition to Thyra's family circle, and reputation-wise, a much better catch than the unfortunate Prince of Orange.