Maybe for his own gain. Kakuzu might see potential in Naruto and think that by training him, he can use Naruto for some future plan. For instance, if Naruto becomes powerful enough, Kakuzu could use him to take on bigger enemies or get access to certain resources.
In a Naruto Kakuzu bloodline fanfiction, it might explore how the two characters' bloodlines interact if they were related in some way. For example, perhaps there was a long - lost family connection that was hidden, and this connection unlocks a new form for Naruto that combines his Uzumaki bloodline with Kakuzu's mysterious lineage. This could lead to Naruto having enhanced physical strength and durability similar to Kakuzu's tough body.
In a fanfiction where Naruto is trained by Kakuzu, Naruto's view of the ninja world might change. Kakuzu has a rather cynical view due to his long life and the many betrayals he has faced. This could either make Naruto more understanding of the darker side of the ninja world or it could make him more determined to change it for the better. On the physical side, Naruto might gain more control over his chakra, as Kakuzu is very proficient in chakra management. He could also learn some new combat strategies that are different from what he has learned from Jiraiya or Kakashi, which would expand his arsenal in battles and make him an even more formidable opponent.
There could be a fanfiction where Kakuzu trains Naruto in dealing with his chakra in a more efficient way. Since Kakuzu has unique abilities related to his chakra and his multiple hearts, he could pass on some knowledge to Naruto about conserving and using chakra better. For example, he might show Naruto how to distribute chakra evenly throughout his body for faster movement or stronger attacks.
One common theme is the exploration of differences. Since Kakuzu in the original is a rather cold - hearted character focused on money, a female version often has different values that clash and then blend with Naruto's ideals of friendship and loyalty.
One possible plot could be that Kakuzu, with his tough and practical nature, trains Naruto in a very different way from the traditional Konoha style. He might focus more on self - reliance and survival skills. Naruto could learn unique jutsu from Kakuzu related to his earth and water - based techniques.
The plot could involve Kakuzu being controlled by a more powerful and evil force. Naruto, after trying to reason with him and failing, has no choice but to kill him. This could also be a part of Naruto's growth, where he realizes that sometimes, in the face of extreme evil, killing is the only option to protect others.
Kakashi could be a significant supporting character. He could provide Naruto with some advice or even join the battle alongside Naruto for a while. Another possible supporting character could be Hinata. Her gentle presence and belief in Naruto could give Naruto the extra motivation he needs during the fight.
In a Wattpad fanfiction where Naruto is trained by Kakuzu, Naruto might learn some unique and unorthodox ninja techniques. Kakuzu is known for his combat skills and his ability to use multiple elements. So Naruto could gain a more diverse combat style, perhaps incorporating some of Kakuzu's stealth and efficient killing moves. However, this would also be a very different kind of training from what he got in the original series, which might change his character development in unexpected ways.