If Henry Frederick Stuart had become King of England instead of his brother, Charles I, things could've been very different.
As the eldest son of King James VI/I and Anna of Denmark, he was raised to be a king. James saw to it that Henry was duly educated and capable, and wanted him to be raised among books and lectures, not around foppish favourites and courtly trifles, both of which his father was indulging in. Henry's star in the Jacobean court rose rapidly. When he suddenly died of typhoid fever at the age of 18 in 1614, the royal family and the country were shocked and bereft.
Henry, however, had quite the temper. He once tried to swing at his father, the king, with a cane, and poked fun at his little brother, for his weak legs and stutter. Charles had lived in Henry's shadow for all of his life, and had received no royal education as Henry did. I'm not saying that Henry would've been the greatest ever monarch, but he definitely would've been a better monarch than his brother. He died so young that it's difficult for us to even begin to judge how his kingship would've played out, but it's probably very safe to say that he would've managed much better than Charles. Yet, Charles also started out being well-liked. Something to keep in mind, I guess.
Who do you think are the greatest monarchs (or monarchs-in-waiting) who never reigned?
Personally, I live Otto von Habsburg (Kaiser Karl's son) due to his advocacy of European integration. Had Austria-Hungary survived, I suspect that he would have made a fine ruler of A-H and possibly helped transform A-H into a mini-European Union.
Both him and his father would have almost certainly seen the survival of the Austro-Hungarian Empire. If their predecessor one of the most unlikable Hapsburgs of the modern era could invoke the loyalty they did, Charles and Otto could have been the most popular monarchs in European history.
I think Prince Charles is awesome and think if he had born in a time where the King had more of a say the world would be a much better place.
Frederick III of Germany I'm assuming doesn't count because he made it to officially become Kaiser but he already knew his death was coming when he became Kaiser.
I was also thinking Frederick III, and I don't care that he technically sat on the throne for a few months. His mortal illness had already rendered him mute so that he could barely communicate. In earlier years he had planned an agenda (make Germany a liberal democracy like Britain) that would take many years to finish but he never had the chance.
Couple of interesting 'what ifs'. If Mary Stuart had been wiser (or luckier) in her choice of husbands, she may have ended up Queen of England, which could have led to some interesting times.
Also Henry Fitzroy. If Henry VIII hadn't had a legitimate son, would he have named his only acknowledged illegitimate son as his successor? Possibly not with Mary and/or Elizabeth in line - but possibly he would. And if he had had no legitimate sons OR daughters, the possibility becomes even more likely.
Would Mary have challenged Henry Fitzroy (if he had lived) upon the death of Henry viii? Very likely. But would she have got the same support to oust a MALE heir - not only a male heir, but the successor designated by her father - even though he was originally illegitimate?
The Black Prince also did have some administrative experience as the Prince of Aquitaine. It was in that capacity that he forged an alliance with Pedro the Cruel and Charles the Bad. Such a great set of names for a triumvirate: Black, Cruel, Bad.
I really am dealing with the unknown on this one, but I do wonder what kind of emperor Constantine's eldest son Crispus would have been like. In 326 Constantine had him executed for unknown reasons, but he had commanded armies since c. 319, and in 324, during the war with Licinius, he commanded the fleet that won the decisive victory in the Battle of the Hellespont. So he seems to have been militarily competent.
Why he should have been executed by his father does open up the possibility that he was too popular with the army and perhaps sought his father's overthrow. That possibility is admittedly a mark against his reputation. Perhaps he felt that his future position was being threatened by his half-brothers Constantinus, Constantius and Constans, who could boast that their mother Fausta was the daughter of Maximian (unlike Crispus' mother Minervina, a woman not known to have been related to an emperor).
With that in mind, he probably would have been no better able to avoid dynastic purges and civil war following his father's death than his half-brothers were.