Naruto could face several challenges. Firstly, the mental strain of controlling such powerful entities. The bijuu have their own consciousness and desires, and keeping them in check mentally can be exhausting. Secondly, there's the issue of mastering different forms and abilities of the bijuu. Each bijuu has unique powers, and learning to utilize them effectively without losing control is no easy feat. Moreover, he may face social challenges as some in the ninja world may not trust him with this much power and try to undermine his control.
In the fanfiction, Naruto controlling the bijuu isn't without difficulties. The bijuu's nature is wild and powerful. Their instincts can be hard to suppress all the time. For example, if a bijuu gets angry during a battle, it might be tough for Naruto to keep it in line. Also, he has to constantly balance using the bijuu's power and his own ninja skills. If he over - relies on the bijuu, he may lose his touch with his own fighting style. And then there's the political aspect. Other ninja villages might see his control of the bijuu as a threat to the balance of power and take actions against him, which is another big challenge for Naruto.
Another issue could be resistance from other forces. There might be some dark entities or villains who don't want Naruto to control the heartless. They could try to stop him, either by directly attacking him or by using their own powers to counteract his control methods.
When Naruto can control the bijuu in the fanfiction, his relationships shift. His friends might be more protective of him as they know his power makes him a bigger target. The villagers who once shunned him could now either idolize him or be even more suspicious. And in the ninja world at large, other villages may try to form alliances with him more eagerly, while some might plot against him out of fear that he'll use the bijuu to dominate. Naruto's newfound control over the bijuu is like a catalyst that stirs up all kinds of reactions in his relationships.
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.
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 possible plotline could be Naruto struggling to come to terms with his half - bijuu nature while also dealing with his relationship with the fem bijuu. Maybe there are other characters who are either afraid or in awe of his unique state, leading to various conflicts.
He could face opposition from other characters who fear his new - found power. Villains might target him to either steal his ability or stop him from using it. Also, his friends might be worried about how this power will change him, which could create emotional challenges for Naruto.