A note on posterities
Of these 15 sovereigns, two were childless (Ludwig II of Bavaria and Karl I of Württemberg, the former in fact never marrying) and the posterity of one (Luís I of Portugal) is entirely extinct. The other 12 all have living descendants today, though only five, Victoria of Great Britain and Ireland, Willem III of the Netherlands, Carl XV of Sweden and Norway, Wilhelm I of Germany and Christian IX of Denmark, can claim present-day sovereigns among them.
George I of Greece will be added to their number when the British sovereign changes, and will in fact be male-line ancestor of what might but probably won’t be called the Second House of Windsor. He is also an ancestor of the Prince of Asturias, so has two crowned descendants to look forward to.
Victoria’s tally includes Elizabeth II of Great Britain, of course, Margrethe II of Denmark, Carl XVI Gustaf of Sweden (twice), Juan Carlos I of Spain and Harald V of Norway. Only Willem-Alexander of the Netherlands descends from Willem III. Carl XV is an ancestor of Margrethe II, Harald V, Henri of Luxembourg and Philippe of Belgium, notably omitting the Swedish monarch.
However Wilhelm I is an ancestor of Carl XVI Gustaf, and likewise of Margrethe II. Like George I he is an ancestor of the Prince of Asturias too, twice in fact, so will one day have a Spanish monarch among his descendants. Finally, Christian IX’s count equals Victoria’s, with Elizabeth II, Margrethe II, Harald V, Henri and Philippe all descendants, and will pass it when there is a new Spanish sovereign, as yet again the Prince of Asturias descends from him.
That leaves six more sovereigns to be covered. Alexander II of Russia’s best hope of a current crowned descendant lies, I would say, in a restoration in Romania or Serbia, both King Michael and Crown Prince Alexander II being descendants. Franz Joseph I of Austria-Hungary I don’t see a tremendous potential; he has a very extensive descent (some of them descendants also of Léopold II of Belgium, whose chances I rate similarly low), but none among them with a throne in even slim prospect. Léopold II is also an ancestor of the present Bonaparte claimants, but I wouldn’t be putting money on a Third Empire.
The hopes of both Vittorio Emanuele II of Italy and Johann I of Saxony rest on the return of the Italian monarchy (either claimant for Vittorio Emanuele II, only the senior albeit disqualified line for Johann I), a very slim chance indeed, though a Bulgarian restoration, a marginally better prospect, would also do the trick for them both. Amadeus I would not be helped by Bulgaria, but could be by Italy. He is in the reverse position to Johann I, as only the Aosta line will do for him.
As it happens the children of Princess Astrid of Belgium descend through their father from all three of Vittorio Emanuele II, Johann I and Amadeus I. Even better, at one time the accession some day of her son Amedeo looked very likely. However the marriage of King Philippe, Princess Astrid’s older brother, and the births of his four children brought that prospect to an end.
I will conclude this note with a brief survey of the posterity of Pedro II, last Emperor of Brazil, whose relationships with his European cousins are now detailed in the post below. His two sons died young but his two daughters both lived to adulthood, married and had children themselves. Descent survives today from both the daughters, that of the elder, Isabel, including an illustrious list of claimants and heirs.
The two pretenders to Brazil, the Orléans pretender to France, the Portuguese pretender Dom Duarte and the sons of Crown Prince Alexander II of Serbia are all of Isabel’s line. There is perhaps more hope that Prince Peter, the eldest of Crown Prince Alexander’s sons, might one day be Peter III, King of Serbia, than there is for the restoration of the other thrones Pedro II's descendants through Isabel lay claim to. As mentioned above, that would also do the trick for Alexander II of Russia, and the two Emperors, Pedro II and Alexander II, both remarkable rulers of vast lands, can reasonably be regarded as joint favourites after the sure thing George I of Greece in my imaginary race for a crowned descendant today among these monarchs of 1871.