In this fanfiction, Naruto may find himself in a situation where he's being hounded by people for various reasons - some for help, some for his power. He simply wants to be left alone to live a normal life. So he might start to avoid the village and the people in it, seeking refuge in a secluded place. There, he could explore his own thoughts and feelings more deeply, perhaps even learning new skills or abilities on his own, away from the watchful eyes of his friends and the village.
Maybe Naruto has had enough of the chaos in the ninja world. He could be tired of all the battles and political intrigue. So, he decides to find a secluded place, perhaps a small island, where he can live in peace and train by himself without any distractions.
It could be someone from the village who doesn't understand Naruto's need for solitude. Maybe a council member who thinks Naruto is shirking his duties and tries to force him back into the spotlight.
Well, perhaps in this fanfiction, Harry has had enough of the constant battles and drama. He just wants a break from his friends too, not because he doesn't love them, but he needs time for himself. He could find an abandoned cottage and start a simple life there, growing his own magical plants and spending his days reading ancient magic texts that he never had time for before. He's basically trying to find peace away from all the chaos.
There's a possibility that the main antagonist is an internal force within Naruto himself. His desire to be alone could be a manifestation of some dark part of his psyche. This 'inner antagonist' could be trying to make Naruto cut himself off from his loved ones completely, leading him down a path of self - destruction. And Naruto has to fight this internal battle while also dealing with any external threats that come his way during his isolation.
Maybe he's frustrated with all the prophecies and his role in the fight against Voldemort. He doesn't want to be constantly pushed into being the hero. He might want some time to figure out who he really is outside of all these expectations. Also, he could be feeling a bit overwhelmed with his studies and the extra - curricular activities like being in Dumbledore's Army. All these things together make him long for some solitude.
The main antagonist could be an internal one - Harry's own guilt. It's like a dark shadow that follows him everywhere. He wants to be alone to escape from it, but it keeps haunting him. It makes him question his every move and decision, and it's this internal struggle that could be seen as the main 'antagonist' in the fanfiction as it constantly opposes his desire for peace and solitude.
Perhaps they would explore their family secrets. Since Naruto has always been in the dark about some aspects of his family, with just the two of them, they could start looking into old family mementos or stories that might lead to uncovering hidden truths.
In a 'naruto fights alone fanfiction', it could be that Naruto is on a mission in a foreign land where he has no support. The local ninjas are all against him for some reason. He has to use his Nine - Tails chakra in a more discreet way as he can't afford to go all - out and draw too much attention. He might befriend some local animals or spirits who help him in a more indirect way, like giving him information about the enemies' weaknesses.
Maybe Naruto is alone because all his friends are with their families. He might be reminiscing about his past Christmases, or perhaps he's on a mission during Christmas and feels a bit lonely while seeing the festive cheer around him.
It could be about Naruto choosing a solitary life after all the battles. Maybe he decides to live alone in the forest to find inner peace away from the chaos of the ninja world.