He could purchase a building that used to be a historical ninja place which he wants to renovate and turn into a place to preserve ninja heritage. He might think it's important to keep the history of the ninja world alive.
Maybe he buys a training building so that he can practice his ninja skills without being disturbed. It could have special facilities for his shadow clone jutsu practice.
It's possible that Naruto buys an apartment building. He might want to provide housing for the orphans or those in need in the village. This would be a very Naruto - like thing to do as he has a big heart.
Maybe he buys an old, mysterious building full of secret passages like a wizarding manor. It could have a hidden attic with ancient spells and a cellar where strange potions were once made.
He might want a place of his own, away from the chaos of Hogwarts and the wizarding world's constant threats. A building could be his private sanctuary where he can relax and practice magic without being bothered.
Perhaps it's a community building. Naruto is a very community - oriented person. He could own a building that serves as a place for the villagers to gather, like a community center. There could be a cafeteria, a place for kids to play, and rooms for meetings and celebrations.
One reason could be to start his own business. Maybe he wants to open a ramen shop of his own, or a place where he can sell ninja tools. After all, he loves ramen and has a good understanding of ninja tools from his own experiences.
One challenge is trust. Not everyone will immediately believe in Naruto's intentions. Some may think he is trying to gain power for himself. Another challenge could be resources. Feeding, housing, and training an army requires a lot of resources. He needs to find ways to get enough funds and supplies.
There could also be external threats. Maybe other villains or rival ninja clans see the apartment building as a target. They might try to take it over or destroy it to undermine Naruto's influence in the village. Naruto would then have to use his combat skills and strategic thinking to protect his property.
In the fanfiction, he might own a building that is actually a magical library. Since Harry has always been interested in knowledge, this building could be full of rare and ancient magic books. It could also be a place where wizards and witches from all over come to study and research, and Harry, as the owner, would be in charge of protecting these precious knowledge resources.
In many Naruto fanfictions, Naruto finding the ANBU building can lead to all sorts of interesting plot developments. Maybe he stumbles upon it by accident while on a mission. He could be curious about the secrecy surrounding it and decide to investigate, which might get him into trouble with the ANBU or even the Hokage.
Perhaps Naruto is being chased by some enemies and he accidentally runs into the ANBU building. This could lead to an unexpected alliance. The ANBU, initially suspicious of him, realize that he is not a threat and together they fight off the common enemies. After that, Naruto could start to have a more positive relationship with the ANBU, which is a big change from how they were before in the fanfiction.
In terms of world - building in Naruto Sonic X fanfiction, the power systems also play a role. Chakra and the Chaos Emeralds could have some sort of connection or interaction. This could lead to new power - up scenarios for the characters. For example, a character could use a Chaos Emerald to enhance their chakra - based attacks. The rules of how these two different power sources interact help shape the new world in the fanfiction.