Well, some fanfictions play with the concept of Naruto and Kyuubi merging in a different way than in the original series. Instead of just using Kyuubi's power, they might merge their consciousnesses. This could result in Naruto having access to Kyuubi's memories and knowledge. Another interesting idea is that Kyuubi could teach Naruto about the other tailed beasts and their true nature. This can open up storylines where Naruto tries to form alliances with the other tailed beasts based on what he learned from Kyuubi.
One interesting concept could be the exploration of Naruto and Kyuubi's relationship beyond the typical master - spirit relationship. For example, in some fanfictions, they might develop a true friendship where they share their thoughts and feelings more openly. Another concept could be the power - sharing aspect. Naruto might gain new and unique abilities by having a deeper connection with Kyuubi. Also, some fanfictions could focus on how Kyuubi's past and its own emotions play into the story, like its regrets or its hopes for the future.
In 'naruto kyuubi doujutsu fanfiction', a cool concept is the potential for new jutsu creation. Since the Kyuubi is a powerful entity, the doujutsu could enable Naruto (assuming he has it) to combine his regular ninjutsu with the Kyuubi's power in novel ways. Maybe he could create a fire - based jutsu that has the added effect of draining the opponent's chakra, as the Kyuubi is known for its voracious chakra appetite. Also, the doujutsu might have a special mode for communication with other tailed beasts.
Well, in this fanfiction, the idea of her relationship with Naruto himself could be really fascinating. Is she a normal daughter - father relationship? Or does Naruto have to constantly protect her from those who fear her Kyuubi - related powers? Also, her training could be an interesting aspect. Who would train her? Would it be Naruto, or some other powerful shinobi in the village? And how would her training differ from the normal ninja training due to her special heritage.
One interesting concept is 'Naruto in a Different Elemental World'. In this, Naruto is transported to a world where the laws of chakra and elements are different, and he has to adapt and learn new skills. Another concept could be 'Naruto as a Teacher'. Here, Naruto has grown up and is now teaching the new generation of ninjas in the village.
One interesting concept could be the power dynamics between Naruto and the Kyuubi in relation to doujutsu. Maybe Naruto gains a unique doujutsu ability because of his connection with the Kyuubi. For example, he could have a doujutsu that allows him to better control the Kyuubi's chakra or even see the memories of the Kyuubi's past hosts.
One interesting concept could be the Kyuubi eyes having unique powers like seeing through illusions that even the Sharingan can't detect. This would give Naruto an edge in battles against powerful foes who rely on deceptive jutsu.
One plotline could be the Kyuubi teaching Naruto some ancient and powerful ninja techniques that were lost over time. Since the Kyuubi has been around for a very long time, it has a wealth of knowledge. Another could be Naruto and the Kyuubi having to team up to face a new, powerful enemy that neither of them could defeat alone.
One interesting concept could be a crossover battle. Imagine Goku's Saiyan powers against Naruto's ninja skills. Another could be a team - up where they face a common enemy that neither could defeat alone, like a super - powerful alien - ninja hybrid. Also, there might be a story about them training together and learning from each other's fighting styles.
The potential for new story arcs in 'naruto x multi fanfiction' is great. There could be arcs where Naruto teams up with characters from different fictional universes to face a common enemy. This would not only test his skills but also force him to think outside the box and learn new strategies from his new allies.
Well, an interesting concept might be the crossover of the ninja villages in Naruto with the military bases in Halo. Imagine Konoha being like a UNSC base, with ninjas using Halo - style dropships for missions. And the idea of Naruto characters using Halo armor for protection while still maintaining their ninja - like agility would be really cool too.
One interesting concept could be the combination of Spiderman's web - slinging abilities with Naruto's ninja skills. For example, imagine Spiderman using chakra to enhance his webs or Naruto using his shadow clones to create diversions while Spiderman attacks from above.