Well, perhaps Percy gets involved in a plot where a powerful enemy from the past, like an old Roman monster that was thought to be defeated, resurfaces. As a senator, he has to use his political influence as well as his combat skills to gather an army and resources to fight this threat. This would involve making deals with other senators, some of whom may have their own hidden agendas, and Percy has to navigate through all this while still being true to his demigod nature.
His character could develop a sense of greater responsibility towards both the demigod and human communities. As a senator, he has to make decisions that affect a large number of people. So, he would grow in his ability to think long - term and consider the consequences of his actions more carefully. He may also become more patient as he has to deal with the slow - moving nature of political processes.
Annabeth would likely be one of his main allies. She's smart and resourceful, and her knowledge of architecture and strategy would be useful in building and defending Rome. Also, Grover could be there. His ability to communicate with nature could help in various ways, like finding hidden passages or getting help from nature spirits.
One possible plot could be Percy Jackson having to navigate the complex politics of Rome as the King. He might face challenges from rival factions within the Roman demigod community who don't want an outsider like him in power. He could also be tasked with protecting Rome from new and powerful monsters that are specifically targeting the city because of his rule.
As Praetor, Percy could be involved in diplomatic relations within the Roman demigod community. He may have to make tough decisions about alliances and resource distribution. Also, his relationship with Annabeth might face new challenges due to the different Roman and Greek cultures. For example, there could be rules in the Roman camp that conflict with their relationship as it was in the Greek world. Fanfiction often explores how they overcome these cultural differences while Percy fulfills his duties as Praetor.
Perhaps the fanfiction could focus on Percy's relationships in his role as emperor. He could form alliances with other demigods in Rome. His relationship with Annabeth might also be affected. She could visit him in Rome and they could work together to solve problems, or there could be tension due to the differences between the Greek and Roman worlds.
Frank Zhang could also be an ally. He is a Roman demigod with shape - shifting abilities. His strength and loyalty would make him a valuable teammate for Percy. Additionally, Nico di Angelo, who has ties to both the Greek and Roman worlds, could be an important ally. His knowledge of the Underworld and his ability to shadow - travel could help Percy in various situations, whether it's getting information or quickly moving around in the Roman camp.
The Roman gods could be antagonists. Since Percy is a Greek demigod, they might not fully trust him as the King of Rome. They could try to undermine his rule, believing that he will favor the Greek side in any disputes or decisions regarding the demigod world.
His control over water could be used for building infrastructure. He could make sure there was a stable water supply for the growing city, which was crucial for its development. Also, his fighting skills and enhanced strength from being a demigod would make him a great leader in battles, protecting Rome from invaders.
One idea could be Percy having to face a new and powerful enemy that is specifically targeting the Roman demigods. He could team up with some of his old friends from the Greek side and form an alliance with the Romans to defeat this threat.