Doing a little follow-up work as I usually do with a challenge, other sovereigns I can think of who shared the distinction are Francesco I of the Two Sicilies; Leopold II of Tuscany; Lodovico I of Parma, who was also Napoleonic King of Etruria; Maria II da Glória of Portugal and her brother Pedro II of Brazil; Friedrich August II (elder brother and predecessor of Johann I) and Friedrich August III of Saxony; and Ferdinand I/V. Ferdinánd and Karl I/IV. Károly of, inter alia, Austria and Hungary. And of course the Emperors Joseph II and Leopold II, but as they were Maria Theresa’s sons that hardly seems to count. Louis XVII of France and Navarre also, but his reign was only titular. As was that of Napoléon II, who likewise would otherwise qualify.
No future sovereign is likely to, barring an unlikely chain of events. As can be seen here, only eight females are alive today that preserve the line and are of an age where future childbearing is possible (modern medical miracles aside), these being Queen Anne of Romania’s granddaughters Karina Medforth-Mills, Angelica Kreuger and Elisabeta-Maria Biarneix, plus Angelica Kreuger’s daughters Courtney and Diana Knight; the sisters Doña Leonor and Doña Micaela de Escoriaza y Gilliéron; and Marie Ghiglione (whose age I am presuming from her parents’ marriage date).
Who knows what King Michael’s version of the Romanian succession is from day to day, or who it will be that reoccupies Romania’s throne should that happy day ever come. I suppose one of the first five listed or the one and only brother of any of them is a chance, as are King Michael's daughters, including Margareta the alleged Crown Princess, but a slim one. Similarly, one of these women and girls might someday marry a sovereign or sovereign’s heir and produce another sovereign, but the likelihood of it seems infinitesimally small.
Maria Theresa’s own uterine line can be traced back as far as an 11th-century Gräfin von Arlon of unknown first name. So Genealogics reckons, anyway; stage I, stage II, stage III. Maria Theresa herself had as many as sixteen children, but descent survives from only four of these, the Emperor Leopold II, Archduchesses Maria Amalia and Maria Carolina, respectively Duchess of Parma and Queen of Naples and Sicily by marriage, and Archduke Ferdinand, who married the heiress of Modena and ought to have been Duke of Modena himself, but due to the Napoleonic Wars wasn’t.
The male line from Maria Theresa and her husband Emperor Franz I is preserved only, albeit abundantly, by the agnatic descendants of Leopold II (no fewer than five of his sons have male-line descendants today), and the uterine line from Maria Theresa only by the uterine descendants of Maria Carolina, not so abundantly as we have seen, a viable line surviving from just one of her daughters. Descent more generally expressed is of course very abundant indeed, the present-day Catholic sovereigns all possessing it, the Prince of Monaco aside.
Felipe VI is descended 13 times by my count, Grand Duke Henri five, Prince Hans-Adam II twice and King Philippe only once. The only (eventual) current heir that will improve on the incumbent’s count is Prince Joseph Wenzel of Liechtenstein, a ten times descendant. Henri is descended from all four of the Imperial children with a surviving posterity, Felipe VI and Hans-Adam II from two of them (the same two, Leopold II and Maria Carolina) and Philippe from Maria Carolina only. Prince Joseph Wenzel again descends from all four.
Finally, Emperor Franz I was overshadowed in life and is in historical memory by his formidable wife, albeit he seemed very content with the marriage and was his wife’s close aide and confidant. One thing about him that stands out to me is his ancestry, which I find to be diverse, fascinating and aesthetically pleasing, and which although he was one of 13 siblings was passed to the royalty of today by him alone in legitimate line.
His younger brother Prince Charles Alexander of Lorraine has a known surviving posterity through an illegitimate daughter, which extends to the children of the Prince of Prussia, of the late Carl, 8th Prince of Wied, and of Archduke Martin of Austria, a grandson of Karl I of that alas former realm. However, all of these descendants of Charles Alexander are descendants also of Franz I and his wife Maria Theresa, with whom Windemere’s challenge began.