There could also be an ancient evil that was sealed away long ago but is now awakened due to the changes in the ninja world. This evil force could possess people or create chaos in the villages. Boruto and Naruto would have to figure out how to stop it, perhaps by seeking out old allies and learning forgotten jutsu.
Perhaps an ancient evil spirit that was awakened due to some of the events related to Naruto's regeneration. This spirit could have its own motives, like corrupting the ninja world and turning it into a place of chaos and destruction.
There are several potential villains. In the Naruto world, Orochimaru would surely be interested in Naruto's new Ultraman powers. He might try to experiment on Naruto to figure out how to obtain those powers for himself. From the Ultraman world, the Alien Mefilas could be a villain. They are known for their cunning and would likely see Naruto as a target. Additionally, some of the rogue ninja in the Naruto world who are power - hungry might team up with the Ultraman villains. They could form an alliance to take over both worlds, seeing Naruto's unique powers as the key to their success.
One potential storyline could be Naruto training Boruto to unlock a new form. This new form could be a combination of Boruto's unique abilities and Naruto's power. Another storyline might be Naruto teaching Boruto how to deal with the pressure of being his son and living up to the Uzumaki name.
Another villain could be a powerful alien race that has invaded Earth X. They are much more advanced than the humans on Earth X and are trying to take over the planet. Alex and Sara would have to find a way to fight them. This could involve Alex using her knowledge of alien technology and Sara's combat skills to find a weakness in the alien invaders.
In a 'boruto shinachiku world fanfic', potential characters would include Shinachiku's family members for sure. Naruto, being his father, would play an important role. Hinata as his mother would be there to support and guide him. Boruto would be his cool older brother. Besides, characters like Konohamaru, who is now a teacher, could also be in the story. He could train Shinachiku and help him grow as a ninja. Also, the friends that Shinachiku makes on his own adventures, new characters created specifically for the fanfic could be part of it.
The Akatsuki could be potential villains. They are always on the lookout for powerful beings or things that can help them achieve their goals. If the OC has something they want, like a special chakra or a unique ability, they would be a threat. Another could be rogue ninja from different villages who see the OC as an easy target or a means to get something valuable.
Maybe there's a group of dark ninjas who are trying to resurrect an ancient evil. They see Naruto and Hinata as the biggest threats to their plan, so they constantly try to attack them and disrupt their lives. These dark ninjas could have some powerful and forbidden jutsu.
Another idea is a 'what - if' scenario. For example, what if Boruto was raised in a world where Naruto had failed to become Hokage? This could lead to a very different Boruto, perhaps more cynical or desperate to change the world on his own. The fanfic could explore how he would grow up and try to fix the situation his father couldn't.
In Boruto x Naruto fanfic, often the character development can be quite fascinating. For example, we could see how Boruto deals with the legacy of his father, Naruto. Writers might create scenarios where Boruto has to face challenges that test his understanding of his father's ideals and his own path. It also allows for a deeper look into Naruto's role as a father and how he imparts his values to Boruto.
One potential villain could be Orochimaru. He might have an interest in Naruto's sister for her unique chakra or some genetic trait. He could be using her as a pawn in his experiments to gain more power or knowledge about the Uzumaki clan.