alternatehistory.com

Besides son Maximilian, later Holy Roman Emperor, and Kunigunde, Duchess of Bavaria, Emperor Frederick III and his wife Eleanor of Portugal had 3 children who died young-Christoph (born in 1455), Helene (b. 1460) and Johann (b. 1466). So what if they all at least outlive their father (so they'd not die before 1493). That means Christoph is obvious candidate to succeede father on Imperial throne, but it is also possible, that he would marry before 1477 (so would not be available for Mary of Burgundy). IOTL Sophia Jagiellon was considered as wife for Max (Matthias Corvinus wanted to marry Hedwig, at the same time (in 1468) he proposed Max-Sophia match, , ITTL Christoph would be 13 and Hedwig 11 at the moment almost mariagable, Matthias OTOH would be refused hand of Casimir's daughter like IOTL, Queen Elizabeth despised him and regarded as usurper and man of low birth, that would not change, but Emperor's son... that is different story). For both sides such match would be desired-Casimir IV would be father-in-law of future emperor, while Habsburgs would strenghten their claims to inheritance of Albertine branch. Helene could marry Vladislaus Jagiellon as part of double match, or George the Rich of Bavaria Landschut (as replacement of his OTL wife Hedwig). So in 1477 only Emperor's younger son Maximilian would be available for Mary of Burgundy. That means likely early split between Imperial and Burgundian branches of Habsburgs. And what about Johann? Would he join clergy?
Top