The plot might center around how Itachi's character changes due to being a Jinchuriki. He was already a complex character. Now, with the added power and burden of the tailed beast, he could become more isolated as he fears losing control. Or, he could form new alliances with other characters who understand the Jinchuriki struggle, like Killer Bee or Gaara. They could teach him how to better manage the power.
One possible plot could be that Itachi's Jinchuriki status is a closely guarded secret at first. As the story progresses, enemies start to suspect something is different about him. This leads to a series of battles and investigations. Meanwhile, Itachi has to train in secret to master the tailed beast's power without drawing too much attention. He might also discover some unique abilities that come with being a Jinchuriki that are different from other known Jinchuriki in the Naruto universe.
Well, the plot could involve Itachi having to deal with the power of the tailed beast within him. Maybe he has to constantly struggle to control it while still maintaining his calm and collected exterior. He could use the power in unexpected ways to protect his village or those he cares about, like Naruto does with the Nine - Tails.
It could be the Akatsuki. Even though Itachi was once part of them, now that he has the power of a Jinchuriki, they might see him as a threat. They could want to capture him to either extract the tailed beast or to stop him from using its power against them.
Perhaps the 5 - Tailed Jinchuriki has escaped from a hidden village and is on the run. Itachi is tasked with finding and capturing them. Karin, who is in the area for some other reason, stumbles upon the Jinchuriki first. She decides to protect the Jinchuriki out of her own sense of justice. When Itachi arrives, instead of immediately attacking, he starts to question his mission. The three of them then form an uneasy alliance to figure out a way to deal with the Akatsuki's plans for the Jinchuriki. Along the way, they face many challenges and their relationships grow. Karin starts to understand Itachi's complex nature and Itachi begins to respect Karin's tenacity, all while trying to keep the Jinchuriki safe.
One possible scenario could be that the 5 - Tailed Jinchuriki has some sort of special power that draws both Itachi and Karin. Maybe Itachi is initially interested in the Jinchuriki for strategic reasons related to his Akatsuki duties, while Karin is intrigued by the unique chakra of the Jinchuriki. As the story unfolds, they all find themselves in a complex web of relationships and power struggles. Itachi might be conflicted between his loyalty to the Akatsuki and his growing feelings towards the Jinchuriki and Karin. Karin, on the other hand, could be using her own abilities to try and get closer to both of them for her own survival and perhaps a bit of romance.
Well, it could be that Itachi trains Naruto to better control his chakra. Maybe Itachi sees potential in Naruto to bring peace to the ninja world and decides to pass on his unique skills like the Sharingan's insights on combat.
A plot could be that Itachi fakes his death earlier and joins Naruto's side. They travel together, with Itachi using his intelligence and skills to guide Naruto in his journey to become Hokage. They encounter various villains and challenges along the way. Itachi's experience and Naruto's enthusiasm complement each other well, making them a formidable team.
Well, it could be about Naruto having to deal with the power of two Bijuus. Maybe he struggles to control their combined power at first. He might face new and more powerful enemies who are after the power he now holds. And there could be a whole new arc of training to master this unique situation.
Well, it could be about Naruto somehow obtaining the power of Son Goku. Maybe there's a strange jutsu that fuses their powers. Naruto could use Son Goku's strength in battles, like having his speed and ki blasts along with his own ninja skills.
A possible plot twist in this fanfiction might be that Naruto's new status as the jinchuriki of Saiken attracts a new group of enemies who are specifically after the power of the Six - Tails. Also, there could be a situation where Naruto's friends start to fear him because of Saiken's power, thinking he might lose control. And perhaps there is a hidden prophecy that only Naruto as the jinchuriki of Saiken can fulfill, which leads to a whole new adventure and set of challenges for him.
In this kind of fanfiction, Naruto might grow up with a better understanding of his Jinchuriki status from the start. He could be trained in a more specialized way. For example, he might learn unique jutsu related to the Nine - Tails' power earlier. Also, he could have more comrades who are also Jinchurikis, and they could form a strong bond, sharing their experiences and difficulties. This could lead to a Naruto who is more confident and in control of his power compared to the original story.
Another plot point could be a love triangle. Suppose there's another character who has feelings for Naruto and is jealous of his growing relationship with fem Itachi. This could create a lot of drama and tension in the story. Also, the backstory of the fem Itachi could be different from the male Itachi's. Her reasons for being involved in certain events in the Naruto world could be a major plot point, like if she has a personal vendetta against a particular group or person.