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.
In some fanfictions, it could be related to an ancient bloodline limit that was dormant in Naruto all along. His Uzumaki heritage might have some connection to water - related powers, and through some sort of emotional or physical trigger, he is able to access and control the ocean.
One challenge could be the over - reliance on his element - controlling ability. He might forget his other skills and strengths. Also, other characters might start to depend too much on him, putting a lot of pressure on Naruto.
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.
One challenge could be loneliness. Without other males around, he might struggle to find that sense of brotherhood or male camaraderie. He may feel isolated in some ways.
He might face strong ocean currents that can push his vessel off course. Also, unpredictable weather like storms can be a huge danger.
Financial difficulties could also be a problem. Naruto might not be well - off initially, and raising a child requires money for things like food, clothes, and education. He would have to find ways to provide for his sister while still being a ninja. Also, there could be threats from Naruto's enemies who might try to target his sister to get to him.
One challenge Naruto might face is Gaara's past traumas. Gaara has been through a lot of pain and isolation, which has made him distrustful. It would be difficult for Naruto to break through those barriers and get Gaara to open up. Another challenge could be the political situation. Suna and Konoha have had their differences in the past, and some in Suna might not be happy with Naruto's influence on Gaara. Naruto would have to deal with any potential political unrest or opposition from those who don't trust Konoha.
Naruto would likely struggle with Hinata's shyness. He is a very outgoing person, and suddenly being in a body that is naturally more reserved could be difficult. For instance, when interacting with other characters, he might accidentally come across as too forward or too quiet, which could lead to misunderstandings. Also, Naruto would have to deal with Hinata's personal relationships. Her friends and family know her in a certain way, and any changes in behavior due to Naruto's possession could cause concern or confusion among them.
He might face powerful rogue ninjas. These ninjas could be after the power of the black fire for themselves. They would see Naruto as a threat and try to take him down to get the power.
One challenge could be controlling the amount of natural energy he uses. If he uses too much, he might lose control and enter a berserk state like when he first tried to use Sage Mode. Another challenge is that his opponents might find ways to disrupt his connection to natural energy sources, leaving him weakened.