One possible plot could be that Percy, through some strange magical or scientific accident, starts to transform into Aku. At first, he might be confused and scared as he feels his body and mind changing. His friends would be shocked and try to find a way to reverse the process. But as he becomes more like Aku, he begins to gain Aku's powers, which he has to learn to control while still trying to hold on to his own moral compass.
It could be a character who wants to use Percy - turned - Aku for their own evil purposes. Maybe a dark wizard who thinks that Aku - Percy can be the ultimate weapon to take over the world. This wizard would try to manipulate Percy's new form and powers to do his bidding.
One idea could be a crossover where Aku accidentally opens a portal to the Naruto universe. Naruto, being the curious and brave ninja he is, decides to explore Aku's world. They could start off as enemies, but gradually come to understand each other's motives and end up teaming up against a greater evil that threatens both of their worlds.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
The character of Aku could be developed in a 'pregnant Aku in Samurai Jack fanfiction' by exploring how he interacts with the world around him differently. He may become more reclusive as he doesn't want others to see his 'weak' state. Or he could become more aggressive in trying to protect his pregnancy, which would be a new aspect of his character. Also, his relationship with his own power might change. Maybe he has to find new ways to use his magic that don't harm the unborn, which would force him to grow and adapt in ways he hasn't before.
One change would be his interests. Instead of just focusing on Greek mythology and his demigod adventures, he now has an additional passion for dragons. He might start spending his free time reading dragon lore.
One idea could be that Percy uses his unique experiences in battles to teach combat strategy in a school for demigods. He could share real - life examples of how he overcame monsters and use those to illustrate key points in strategy.
Well, if Percy Jackson were to become an Avenger in fanfiction, there would be some interesting dynamics. For one, his loyalty to his demigod friends might conflict with the Avengers' way of doing things at times. But his ability to think on his feet, which he developed from all his quests, would help him adapt quickly. And his relationship with the other Avengers, like maybe a friendly rivalry with Thor over who has better control over elemental powers, could be a great plot point.
I'm not sure who Aku specifically is. Aku could be a person with a great imagination who tells all kinds of stories, like adventure stories, fairy tales or even true - life stories.