Great male heirs that could have been

In most societies succession has been patrilinear, going from Father to son, thus lack of a male heir could doom the hard fought legacy of any great king and dynasty. So today I'm asking you which male heirs that never were could have changed history.
 
Fernando de la Cerda (1255 - 1275), son of Alfonso X of Castile

Martin I of Sicily (c. 1374/1376 – 25 July 1409), son of King Martin of Aragon.
 
I often wonder what if either Otto III or Henry II had a son. The Ottonians were the Capetians of Germany, but they never consolidate as a dynasty thanks to them lacking a male heir.
After them, the following dynasties started to lose power to the Church, the Dukes and the minor princes, ending in the mess we now know and love.
 
Hard to say how good or bad he would have been, but him living probably beats so many years of war with England (and certainly changes history):

Alexander son of Alexander III of Scotland, died a week after turning twenty (1264-1284).

Valdemar the Young (1209-1231), son of Valdemar Ii of Denmark might be another interesting one as far as avoiding a lot of trouble - potentially, at least.
 
Baltasar Carlos of Spain. A dynamic capable ruler to succeed Philip IV could have perhaps initiated an earlier recovery of Spain and preserved the Spanish empire.

Sigismund Casimir Vasa. Not sure if this counts since the Polish throne is elective and its not guaranteed that he'd be able to succeed his father but there was potential there and perhaps the deluge could have been avoided and the Vasa line continued.
 
Juan of Castile (1355-1405), natural son of Pedro I of Castile.

Carlos of Viana (1421-1461), son of Queen Blanche of Navarre and King Juan II of Aragon.

William Ætheling (1103 – 1120), son of Henry I of England.
 
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Alexander II's oldest son Nicholas would had made much better tsar than his OTL successors. Russia would had transferred to consittutional monarchy and avoided all of that shit what has faced through 20th century.

Dauphin Louis, son of Louis XV and father of OTL Louis XVI would had been intresting to keep alive and inherit his father.
 
William Ætheling (1103-1120), son of Henry I of England (no Anarchy)
Philip of France (1116-1131), son of Louis VI of France (no Louis VII)
Arthur I of Brittany (1187-1203), grandson of Henry II of England (no Capetian Brittany, longer-lived Angevin empire?)
John I of France (1316), son of Louis X of France (no Hundred Years War, no Salic Law)
 
There are so many options during the late 15th and early 16th century to go with.

Arthur, Prince of Wales, Henry, Earl of Cornwall for England. Even Edward V of England.

For France, Charles Orlando born in 1492. Francis III of Brittany is another.

And my beloved Trastamaras and Avis got screwed really hard. Alfonso, Prince of Portugal. Juan, Prince of Asturias, Miguel da Paz. And even if she's not a man, I would say Isabella of Aragon could have been a great ruler if she had not become such a tragic figure.
 
Alexander II's oldest son Nicholas would had made much better tsar than his OTL successors. Russia would had transferred to consittutional monarchy and avoided all of that shit what has faced through 20th century.
Agree to disagree. Alexander III was the emperor Russia needed, his brother had too many of his dad's jingoistic/wishy-washy/mealy-mouthed ideas AIUI, so would likely have got the empire embroiled in all sorts of scrapes and the monarchy might have been in worse straits far sooner. After all, as bad as Alexander III may have been, he at least kept Russia at peace and there was more industrial development under him than in his dad's reign IIRC.
 
Crispus son of Constantine the Great. Seems like he was a very effective commander even at such a young age. I imagine he’d of been a good emperor and I think give that and him being a fair bit older that he would’ve been more secured in his reign and the civil wars and infighting that followed after his father died would be avoided or lessened. Perhaps if he could rule for a decent amount of time and pass things off to a son of his own or something then a longer lasting Constantinian dynasty could’ve been established. Which I imagine would’ve really helped the empire.
 
Alphonso of England, the third son of Edward I, had managed to live to 11 before he suddenly died of disease in 1284. Edward and Eleanor of Castile appear to have had big plans for him and from what I've read, Eleanor was especially making sure Alphonso would be well-educated. He was betrothed to Margaret of Holland, whose brother eventually ended up marrying Alphonso's sister to maintain the alliance between England and Holland after Alphonso's OTL death. To be sure, Alphonso is essentially a cypher, but he also would have enjoyed the extensive education Eleanor had planned for him and would have come of age nearly 20 years before Edward's OTL death in 1307. At any rate, he might have been a far more effective king than his younger brother Edward of Caenaerfon.
 
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Alphonso of England, the third son of Edward I, had managed to live to 11 before he suddenly died of disease in 1284. Edward and Eleanor of Castile appear to have had big plans for him and from what I've read, Eleanor was especially making sure Alphonso would be well-educated. He was betrothed to Margaret of Holland, whose brother eventually ended up marrying Alphonso's sister to maintain the alliance between England and Holland after Alphonso's OTL death. To be sure, Alphonso is essentially a cypher, but he also would have enjoyed the extensive education Eleanor had planned for him and would have come of age nearly 20 years before Edward's OTL death in 1307. At any rate, he might have been a far more effective king than his younger brother Edward of Caenaerfon.
 
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