He might face powerful enemies. For example, if the enemies are from a secret organization with strong and unknown jutsu, Naruto would have to quickly figure out their weaknesses. This could be difficult as he has no prior knowledge of them.
The environment could be a challenge. If Sakumo is trapped in a dangerous place like a collapsing cave or a poisoned swamp, Naruto has to not only deal with the enemies but also ensure Sakumo's safety in such a hazardous environment.
One challenge could be Naruto's impulsive nature. He often rushes into things without thinking, and Tsunade would have to find a way to rein him in. Another might be dealing with Naruto's fame. Since he is the Jinchuriki of the Nine - Tails, he has a lot of attention on him, and Tsunade would need to protect him from those who might want to use or harm him.
The villains might use the baby as a human shield. Spiderman would have to be really careful not to harm the baby while trying to get past the villains. He'd have to use his wits and his web - slinging skills in a very precise way to disarm the villains without putting the baby in danger.
He might face powerful enemies. Maybe a new pirate crew with a devil - fruit user who can nullify Luffy's powers. Luffy would have to find a way to overcome this power - nullifying ability to reach Ace.
One challenge could be the physical strain. The Harashin technique might require a different kind of chakra control and physical movement than what Naruto is used to. For example, it could demand more precise and rapid movements, which might be difficult for Naruto at first.
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.
One challenge could be communication. If they come from different backgrounds or cultures within the fanfic world, they might have trouble understanding each other at first. For example, if Sayo Hitsugi has a very formal and traditional way of speaking and Naruto is his usual brash self. Another challenge could be trust. Naruto has been betrayed before, so he might be hesitant to trust Sayo Hitsugi right away, and vice versa. They would need to overcome this to work together effectively.
Sasuke might face the challenge of understanding baby Naruto's needs. Since he's not used to dealing with infants, things like feeding, changing diapers could be really difficult for him.
Well, for Naruto in this fanfiction, a major challenge would be dealing with the recoil of Raikiri. Since it's a powerful jutsu, it likely has a significant recoil effect on the user. Naruto would need to learn how to manage that so he doesn't get hurt every time he uses it. Also, in the world of fanfiction, there could be other ninja who don't want Naruto to learn Raikiri. They might try to sabotage his training, which would be an external challenge he has to face. And let's not forget that Naruto has his own unique fighting style. He would need to find a way to integrate Raikiri into his existing style without losing his edge in combat.
Another challenge would be the level of power they start at. Gohan has extremely high power levels as a Saiyan, especially if he has access to his Super Saiyan forms. Naruto, while very powerful in his own right, might find it hard to keep up at first. This could lead to feelings of inadequacy on Naruto's part and pressure on Gohan to not accidentally harm Naruto during training.