Well, you could focus on the relationships between the bijuu themselves in a Naruto multiple bijuu fanfiction. Maybe some of the bijuu have old grudges or alliances that affect how they interact when multiple are involved. Also, consider a plot where a new villain emerges who has the power to corrupt the bijuu. This would force the characters in the fanfiction to find ways to purify the bijuu while also dealing with their combined, but tainted, power. Or, perhaps a time - travel element where a character goes back in time to prevent the separation of the bijuu in the first place, leading to a completely different Naruto world with multiple bijuu in different situations.
In a Naruto multiple bijuu fanfiction, you could explore the cultural impact of having multiple bijuu in one place. How would the different ninja villages react? Some might see it as a threat and try to isolate or eliminate the person or group with multiple bijuu. Others might try to form alliances. You could also have a story where the multiple bijuu start to merge their powers in unexpected ways, creating new forms or abilities. And then there's the idea of a character who can communicate with the bijuu on a deeper level than Naruto, and uses this ability to bring about peace among the ninja world by leveraging the power of the multiple bijuu in a more diplomatic way.
One interesting idea could be the exploration of new powers. When Naruto merges with the Bijuu, he might gain unique abilities that are a combination of his own skills and the Bijuu's powers. For example, he could have enhanced strength and speed, along with the ability to use new types of jutsu that are based on the Bijuu's nature. Another aspect could be the impact on his relationships. His friends might view him differently, and he would have to deal with the newfound power and the responsibilities that come with it.
In fanfiction, if Naruto is a jinchuuriki of multiple bijuu, it could mean extreme power levels. He might be able to access a wider range of abilities from different bijuu. For example, he could combine the fire - based attacks of one bijuu with the water - based defenses of another. This would make him an almost unbeatable force in battle scenarios.
Naruto would be an obvious choice. Since he already has a connection with the Nine - Tailed Fox, exploring how he expands that connection to other bijuu could be really interesting. He has the willpower and the heart to handle multiple powerful entities within him.
Well, one interesting idea could be the unique combinations of Naruto's abilities with multiple zanpakuto powers. For example, if Naruto could use the speed and cutting abilities of a zanpakuto in his combat style, it would add a whole new dimension to his fighting. Another idea might be the different forms of the zanpakuto that Naruto could unlock, similar to how he has different forms of his own jutsu.
One interesting plot idea could be that the Fem Kyuubi and Fem Bijuu form an alliance to protect Naruto from a new, powerful enemy. They use their unique powers and femininity to outwit the foe. For example, they could use their charm to distract the enemy while Naruto prepares a powerful jutsu.
One interesting concept could be the unique powers that the artificial bijuu possess. They might have powers that are a blend of different existing bijuu, creating new and unexpected abilities.
New relationships could be very interesting. He could become like a big brother to the bijuu spirits. Naruto's kind nature would make him try to understand and protect them. The bijuu might start to see him not just as a host but as a true friend. They could share their stories and experiences with him, and in turn, Naruto could use their knowledge to grow as a ninja and a person. This would be a really unique bond compared to his relationship with just the Nine - Tails.
One interesting plot point could be Naruto having an even more enhanced connection with the bijuu, allowing him to access new and powerful forms. For example, he might be able to combine the powers of all the bijuu in a unique way that no one has ever seen before.
One interesting plot could be the main character traveling back in time and preventing the initial capture of the bijuus. This would completely change the power dynamics in the Naruto world. Another could be a character using time travel to team up with past jinchuriki to fight a new, greater threat.
In fanfiction, if Naruto is half - bijuu, it could mean he has enhanced physical abilities. For example, he might have greater strength and speed like a full - bijuu. His chakra reserves could also be much larger, allowing him to perform more powerful jutsu for longer periods.