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Given the following post, respond as if you're commenting on Reddit:
human
[Game of Thrones] What if Aemon Targaryen became the king. (Spoilers)

So what if Aemon took up his duty as the king and left the nights watch. Assuming he faced no repercussions for leaving the watch, how would things be different if he was the king and not the Mad King. Would Lyanna Stark still been taken by rhaegar targaryen?
gpt
Even the very wise cannot begin to fathom the twist and turns of fate.  Perhaps Aemon would've failed to have children or they would have died early and the throne would have passed to Egg regardless.

Perhaps the tragedy at Summerhall wouldn't have happened, or maybe it would anyways.

I will tell you this.  We once had an heir.  Prince Baelor Breakspear.  Bloody good man he was.  Set to become King, and a great one.  Then one day he stands up for a Hedge Knight in a tournament against his kin and his brother smashes his skull open with a mace.  He left a son, but his son died in the great spring sickness and his sons were stillborn.  His entire lineage was wiped, despite being so strong and sure, and the throne passed to a lesser branch of House Targaryen.

Perhaps much would've changed, perhaps nothing.  Perhaps Aemon would've become a bad king, as the Throne was offered to him when he was largely in his youth as a Maester, before he took the Black.  He took the Black explicitly to keep them from offering the Throne to him again.

People who know Aemon would say he was a good man, and by some definitions that means he would be a good King, but there's no certainty but death and winter.