Rise of the House of Hunyadi
France-England was a nation of immense power and wealth. Yet in the late 15th century, the name universally associated with power and glory was not King Henri II, but Matthias Hunyadi, known as Corvinus, the King of Hungary. Corvinus was elected as a puppet ruler by his nobles, who thought the naive boy-scholar would be easy to manipulate; he was anything but. Assembling an army of mercenaries, with a Hussite core, that would become feared under the name of “the Black Army”, he secured his own power over the aristocrats, and then moved abroad for further prey. In an unrivalled military career, Corvinus wrested Bohemia, Moravia and Silesia from the Jagiellons and their allies, tore Austria and Carinthia, Styria and Carniola from the Hapsburgs, sending the Emperor scurrying from conquered Vienna, and then turned south to repel, and then to hammer back, the incursions of the Turks, liberating Bosnia and Moldavia. Nor was he only a horseback prince; a scholar to the end, he assembled the first and largest secular library of the renaissance, second only to the Vatican (which it surpassed in secular and vernacular tomes), and sought to make Hungary the capital of European culture, inspiring many imitators in the process – most notably Cosimo de Medici.
In 1476, Corvinus married the sister of René II of Aragon, Naples, Sicily, Lorraine, Bar, Anjou, Vaudémont and Provence. It was an obvious match: René was in his own right one of the most powerful men in Europe, and a key ally of the King in Paris; he also provided a firm ally at Corvinus’ back. By her, he had both a son and a daughter; when he died in 1490, his son, John Hunyadi, assumed the throne of Hungary, although a minor, and with the help of his bastard brother (also John), his mother and her allies across the sea, and his father’s loyalists in the Black Army, he suppressed insurrectionary efforts on the part of his nobles.
The Hapsburgs, meanwhile, largely dwindled away. Although Emperor Frederick was able to secure papal election for his son, Maximilian, it was the last flourish of a bright but brief dynastic power: after the fall of Vienna, they were left with nothing but the Tyrol and some holdings in Breisgau and Sundgau, and some claims over parts of the Swiss territory.
[ FFAOT: the first paragraph is true. Corvinus, however, married the daughter of our King of Naples (a worse match than in this timeline!), and had no children by her. He declared his young, bastard son to be his heir, but his nobles refused to accept an ill-born king, and overthrew him with the aid of the Hapsburgs. The Black Army then revolted (partly to support Corvinus’ heir, and partly because the nobles insisted on slashing its pay and downsizing it), but without effective leadership or any real hope of political success, it became a marauding force of unprecedented scale. Between corrupt and useless barons and a countryside in chaos, Hungary collapsed from Great Power to nothingness, ending up shared between the Hapsburgs and the Turks.
Its far from certain that a legitimate male heir could have changed this; but I think it is certainly possible that a legitimate heir, combined with powerful overseas allies, could have held the boyars in check long enough for the dynasty to become established.
The Hapsburgs, meanwhile, rose dramatically to power, but we often overlook how improbable that rise really was. Only the collapse of Hungary (allowing them to retake Austria) and the marriage of Mary of Burgundy to Maximillian, allowed them to survive their first few generations as a mighty house.]