At the risk of sounding like a total Hapsburg nut, I was just wondering that because of the Diplomatic Revolution, Empress Maria Theresa pulled off five Hapsburg-Bourbon marriages - her son, the Emperor Josef II and the Archduchess Maria Amalie to the son and daughter of the duke of Parma; the Emperor Leopold II and first Archduchess Maria Johanna, then Maria Josefa and finally Maria Karoline to the son and daughter of the king of Naples (later the king of Spain); and the most famous Hapsburg-Bourbon match of all - Maria Antonia to Louis XVI.
Now, Maria Johanna and Maria Josefa, as well as their brother, the Archduke Karl, died from smallpox. If Karl had survived Leopold and not Max wouldve been pushed into the cloth of the clergy. But say by some fluke - not like their sister Elisabeth who survived but was so scarred by smallpox her marriage negotiations AFAIK with the widowed Louis XV were broken off - survive, or better yet don't get smallpox at all (the Empress was a big supporter of innoculation). And while were at it, what about a butterfly going to Versailles to let Mgr le Duc de Bourgogne not die, and maybe let Mgr le Duc de Berri (Louis XVI) die instead????
Also, if their sisters don't die, it rules out sending Karoline to Naples and Antonia to Paris, and with 2 sisters (Archduchesses Maria Anna, and the aforementioned Elisabeth) already sitting as Abbesses, and Maria Theresia's obsession with having as many grandchildren as possible, I don't see either archduchess being a case of "get thee to a nunnery". So who would Theresia's three youngest children - Karoline, Antonia and Max - marry in this situation?