Well, in the 'Percy becomes the king fanfiction', it could be that Percy uses his unique skills and experiences. Maybe his ability to lead his friends in tough situations translates into leading a kingdom. He might also introduce new and more just laws, inspired by his own values of loyalty and fairness.
In this fanfiction, Percy might face a lot of challenges on his way to becoming king. He could have to prove himself to the people, deal with political intrigue, and perhaps even face some old enemies who don't want him in power.
There are many possibilities in the 'Percy becomes the king fanfiction'. Percy could start by uniting different factions within the kingdom. He might have to make difficult decisions regarding the economy, like how to deal with poverty or manage the kingdom's resources. Also, his relationships with other characters could change. His friends could become his advisors, and he might have to marry for political reasons, which would bring a whole new set of storylines and character developments.