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.
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.
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.
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.
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.
In many Naruto fanfictions, if Naruto has three bijuu, they could be the Nine - Tails (Kurama) which he already has in the original series, perhaps the Eight - Tails (Gyuki) as it has a connection with Naruto's world and Akatsuki's hunt for the bijuu. And another could be the Three - Tails (Isobu). However, it really depends on the specific plot and creative direction of the fanfiction.
It could be an ancient sage. In Naruto lore, there are sages who have had a great influence on the world. An ancient sage could be the one who created or fathered the bijuu. Maybe he had some great power and knowledge that allowed him to bring the bijuu into existence.
It could be Shukaku. Since Shukaku was one of the first bijuu introduced in the series and has a sand - based power, it would be an interesting addition to Naruto's arsenal. Naruto's positive and energetic personality could also have an impact on Shukaku's usually grumpy and solitary nature.
Well, having two bijuu would likely make Naruto extremely powerful. He could potentially combine their unique chakra natures and abilities in combat. For example, if one bijuu has a fire - based ability and the other has a water - based one, he might be able to create new and unexpected jutsu by mixing them. Also, it would change his standing in the ninja world. People would probably be more in awe or more fearful of him, depending on their stance towards Naruto.