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.
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.
There could be their children's friends. Maybe a young ninja from a far - off village who comes to study in the Hidden Leaf. His unique abilities and different cultural background could bring new elements to the story.
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.
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.
In fanfiction, Naruto, Hinata, and Ino could interact in a training scenario. Naruto could be helping Hinata and Ino improve their combat skills. He would encourage Hinata to be more assertive in her attacks, while also teaching Ino some new chakra - control techniques. This interaction would not only strengthen their fighting abilities but also deepen their friendships.
Tsunade could be part of it. Since she is a Senju, she could be a powerful ally and also someone who helps Naruto and Hinata navigate the complex relationships in the harem. Also, Nawaki Senju could be brought back in some way, maybe as a spirit or through some time - travel plot device. He could befriend Naruto and be involved in the growth of the relationships within the harem.
Maybe a young apprentice of Hinata. A girl who is inspired by Hinata's grace and strength. This character could bring a fresh dynamic as she looks up to Hinata and also gets to know Naruto. Additionally, there could be a mysterious figure from a far - off land who comes to the Hidden Leaf and has an unexpected connection to Naruto and Hinata's story.
Another type of new character could be a mysterious figure who appears in Naruto's future vision. This character might have powers that are beyond comprehension at first. They could be a key to unlocking a new form of chakra or a secret about the origin of the ninja world. Their motives would be unclear at first, adding an element of mystery to the fanfiction.
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.
It could be some new group that emerged due to the changes in the ninja world over time. They might oppose the way Naruto and Hinata's village is being run and want to impose their own ideals.