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.
One possible plot could be that Aku somehow ends up in the Naruto universe. Naruto, being a fan, is initially excited but then realizes that Aku's presence is causing chaos. So, Naruto has to find a way to either send Aku back or find a way to co - exist with Aku without letting Aku's evil influence overtake the ninja villages. He might try to reform Aku using his own positive attitude and the power of friendship, which is a common theme in Naruto.
Since Naruto has the Nine - Tails' chakra, which is a powerful and primal energy, and Aku has his own otherworldly and malevolent energy, they could combine these energies in a last - ditch effort to defeat an ultimate enemy. This combination could create a completely new form for Naruto, one that has the strength and ferocity of the Nine - Tails along with Aku's dark and mysterious powers. It would be a visually stunning and very powerful transformation that could turn the tide of a great battle.
Well, a plot could revolve around Aku trying to keep his pregnancy a secret from Samurai Jack. He might be worried that Jack would use this new situation to his advantage. And then there could be a sub - plot about Aku having strange cravings that lead him to unexpected places in the world of the fanfiction, causing chaos in different ways.
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.
Aku could be a storyteller. Maybe it's a person's name.
One idea could be that the Sword of Zen Aku is hidden in an ancient and forgotten chamber in Hogwarts. Harry discovers an old map or prophecy that leads him to it. Another idea is that the sword has been stolen by a dark wizard and Harry has to track it down. Maybe the sword has the power to control certain magical elements, like the ability to stop a dark curse that's spreading across the wizarding world.
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.
In a Naruto masterchef fanfiction, Naruto could compete in a cooking competition. He would bring his unique style which combines his love for ramen and his ninja spirit. His opponents might underestimate him at first because he seems so unconventional. But then he wows everyone with his creative dishes that are inspired by his friends and their different abilities.
Perhaps in a fanfiction, Kairi somehow ends up in the Naruto universe. She is mistaken for a foreign ninja at first. Kairi tries to find a way back home while also getting involved in Naruto's adventures. She helps Naruto and his friends in some battles with her unique keyblade abilities. Along the way, Naruto and Kairi share stories of their respective worlds and form a deep bond.