Guillaume IV could have decided that his first cousin was dynastic and as such his heir in Luxembourg the same applies to Prince Nikolaus had he survived and succeeded his nephew.
The son married a daughter of Tsar Alexander II from his second morganatic marriage.
Im not sure if it had been a popular move or one accepted by for instance Wilhelm II of Germany. With their dwindling powers at the end of the 19th and early 20th century the various Royal Houses were holding on to their strict rules surrounding dynastic marriages.
Had he survived his half-nephew, Grand Duke Guillaume IV, as the sovereign grand duke of Luxembourg, is it certain that his own successor would have been his morganaut son, Georg Merenborg?
He had married Natalia Pushkina morganatically in 1868, after the house was deposed but before it came to the throne of Luxembourg. Regardless of the circumstances, it should have been clear enough that all issue born of the union should have been excluded from the grand ducal succession, should the house of Nassau-Weilburg come to reign there.
But I can't imagine an hypothetical reigning Grand Duke Nikolaus (as long as there were any dynastic males around, no female could succeed) recognizing any dynastic female as his successor, and accepting the exclusion of his own son -- notwithstanding the fact that Georg was not born of an approved dynastic marriage.
It's difficult to imagine the woman we know in history as the Grand Duchess Marie Adelaide as the eventual successor, under the hypothetical scenario whereby the half-brother of her paternal grandfather, Grand Duke Adolphe, DID survive to occupy the throne. After all, in a morganatic marriage, only the issue born to the union are affected: the dynast himself does not forfeit his own succession rights.
And faced with extinction in the male line, the house of Nassau-Weilburg might very well have been in a position to de-morganatize the marriage of Nikolaus -- which seems plausible, had he been on the throne.
After all, Grand Duke Guillaume IV was able to take actions to ensure the succession of his own daughters, by virtue of occupying the said throne -- and fortuitously enough, after his half-uncle died. So he was able to clarify the terms of the Nassau Family Pact and have Luxembourg's chamber of deputies pass the 1908 Family Statute. What that measure did was exluded the Merenborgs and confirm the status of his eldest daughter as heiress presumptive to the throne.
But things could have been different, had Nikolaus survived: he would have succeeded as the reigning grand duke. The question is: could he (and would he likely) have de-morganatized his own marriage?