Maybe he could gain the ability to transform into an Egyptian animal, like a falcon or a jackal. This would be a cool addition to his existing powers and would give him new ways to move around and fight enemies.
One way could be through magic systems. Maybe Percy's water - controlling powers could interact with Egyptian magic in some unique way. For example, the Egyptian magic related to the Nile River could be linked to Percy's abilities.
I really like 'Percy and the Magic Arts'. In this fanfiction, Percy discovers he has a hidden talent for magic in addition to his demigod powers. The author does a wonderful job of exploring how he would balance these two aspects of himself. It's full of action, with Percy facing off against magical creatures that are new even to him.
In a more complex way, Percy's interactions could be influenced by the political situation between the Greek and Egyptian gods. If there's some sort of tension between the two pantheons, the demigods might be caught in the middle. Percy would have to be diplomatic and try to find ways to keep the peace while still maintaining his own beliefs and loyalties. For example, if there was a dispute over a powerful magical artifact, Percy and the Egyptian demigods would have to figure out how to resolve it without starting a full - blown war between their respective godly parents.
Percy would interact in a cautious yet curious way. He'd be curious about their powers, like Anubis' role in the afterlife. But he'd also be cautious as he doesn't know how they'll react to a Greek demigod. He might use his charm, which has worked on some Greek gods, to try and build relationships. However, he may face some resistance from the more xenophobic Egyptian gods who see him as an intruder.
Percy would probably be initially cautious when interacting with Egyptian gods. For example, when meeting Ra, the sun god, Percy might be in awe but also a bit hesitant as their powers are so different. He would need to show respect as he does with the Greek gods. The Egyptian gods, in turn, might be curious about Percy's abilities, especially his control over water which is not as prominent in Egyptian mythology.
Another storyline might involve Percy getting lost in the Egyptian afterlife. He has to navigate through the different realms with the help of Egyptian gods such as Thoth, who is known for his wisdom. Along the way, he discovers secrets about the connection between the Greek and Egyptian pantheons. This could include how their powers are related and how the two mythologies might have influenced each other in ancient times.
In a fanfiction where Percy Jackson meets the Egyptian gods, it would be a clash of two mythologies. Percy's sword skills and ability to control water might not be as useful as he thinks against the Egyptian gods' unique powers. He could befriend some of the Egyptian gods who share similar values to the Greek gods he knows. However, he might also make enemies among those who see him as an outsider. The fanfic could explore how Percy uses his wits and his knowledge of Greek mythology to navigate the world of Egyptian gods and their associated creatures.
Set, the Egyptian god of chaos. He could be a great antagonist as he might see the combination of Greek and Egyptian powers in Percy and the other demigods as a threat to his own plans of spreading chaos. He could try to manipulate events to turn the two groups against each other.
You can start by looking on fanfiction websites like Archive of Our Own. There are a lot of talented writers who post their 'Percy Jackson Magician Fanfiction' there. Just search for the relevant tags and you'll find a whole bunch of stories.