Well, in some fanfictions, Naruto might defeat Kisame in a battle and take the sword as a trophy. Maybe he uses his newfound Sage Mode powers or some other advanced jutsu to overpower Kisame and claim the sword.
Another possibility is that Naruto stumbles upon Kisame's sword after Kisame has been defeated by someone else. Naruto, being the curious and adventurous type, decides to take the sword with him, thinking it could be useful in his future battles.
However, it might also attract unwanted attention. Other villains or even some of the shinobi villages could see Naruto with Kisame's sword and assume he has some sort of alliance with Kisame or the Akatsuki. This could lead to him being targeted or distrusted by others.
Maybe he defeated Zabuza in a battle and took it as a trophy.
Kisame's sword could be a plot device by having it hold the essence of an ancient ninja art. Naruto, in his journey to become stronger, discovers that the sword can teach him this art. He has to first overcome the dark nature of the sword, which was influenced by Kisame. So, he battles with the sword's influence while trying to learn the art, creating a complex and engaging plot.
One interesting plot could be that Naruto discovers a hidden power within Kisame's sword. Maybe during a battle, the sword starts to react to Naruto's chakra in an unexpected way, leading to a new form of combat for Naruto.
The plot could involve Naruto getting the sword as a gift from an unexpected source. With Kisame's sword in his possession, Naruto becomes a target for many who either want the sword for themselves or fear the power it could give Naruto. Naruto then has to protect the sword while also trying to master it. He might seek out old sword - masters in the ninja world to train him. Along the way, he could face off against enemies who are also after the sword, and his friends might be in danger because of it.
In some fanfictions, Naruto might obtain Cloud's sword through a series of adventures. Maybe he helps the Cloud Village in a great battle and as a reward, he is given the sword. Or perhaps he discovers it during an exploration of an ancient ruin that has a connection to the Cloud Village's history.
Kisame would probably be very angry at first. Samehada is his weapon and has been with him for a long time. He might feel betrayed and try to get Samehada back by force.
Iruka could be an important character. He's always been like a father figure to Naruto, and now with this new revelation, he might have to help Naruto deal with it. Hinata could also be in the story, and her feelings for Naruto could be complicated by his connection to Kisame. And then there could be some original characters created just for this fanfiction, like a ninja who was a former ally of Kisame and has some information for Naruto.
There could be. Fanfiction is a vast realm where all sorts of ideas are explored. However, I'm not sure if there is a highly popular or mainstream 'naruto is kisame apprentice' fanfiction. It might be a more niche concept, but it's entirely possible that some creative writer has delved into this idea to create an interesting story about Naruto learning from Kisame, perhaps incorporating Kisame's unique fighting style and view of the ninja world into Naruto's growth.
It would make Naruto more versatile. He's always been known for his straightforward and energetic fighting style with his ninjutsu. Having a sword like Kisame's would add a more serious and perhaps a bit of a dark edge to his character. He would have to be more calculated in his attacks, as a sword requires different combat tactics compared to his usual jutsus.