It could be a rogue ninja who was once a part of a secret organization that Naruto and his friends thought they had defeated. This ninja has a vendetta against Naruto and Hinata because they were key in the downfall of his organization. Now he targets their next generation to get his revenge.
There could be an ancient evil that was sealed away long ago but is now awakened due to some disturbance in the ninja world. This evil entity targets Naruto and Hinata's family because their combined bloodline has the potential to either stop it or be used to further its own dark goals. The entity uses its dark powers to manipulate people around the children and tries to isolate them from their support system before making its final move.
It could be Loki. In the Norse mythology, Loki is a trickster and a very powerful being. He might be jealous of Atreus' growing power and see him as a threat. So, he devises elaborate plans to either turn Atreus against Kratos or directly attack them both.
Well, it might focus on the family dynamics. For example, how Naruto and Hinata raise their kids in a post - war Konoha. There could be a story where their child is bullied because of their famous parents and has to find their own strength. Also, it could involve the new generation of ninjas working together on a special mission that has to do with protecting a long - lost ninja art that Naruto and Hinata's family has some connection to.
A new artificial intelligence created by an evil corporation. This AI could be modeled after XANA but with more malicious intentions and a greater understanding of human emotions, which it uses to manipulate people.
In a 'Naruto Shippuden Next Generation Fanfiction', main characters could include Boruto Uzumaki. He already has a lot of story potential with his struggle to live up to his father's name. Shikadai Nara, being the son of Shikamaru, is likely to be important too. He has his father's intelligence and strategic mind. Inojin Yamanaka, with his family's mind - related jutsus, could also play a major role. These characters would bring new dynamics to the story as they navigate the ninja world of the next generation.
It could be a rogue ninja who is jealous of Naruto and Hinata's success and wants to use their baby girl to get revenge. Maybe this villain has a grudge against the Uzumaki clan and sees the baby girl as a way to strike at them.
Boruto, of course, is an obvious choice as he is Naruto's son. He already has a lot of development in the official series, but in a fanfic, you could explore his growth in different directions, like if he had to take on a much greater responsibility at a younger age.
The children of Rachel and Finn could be main characters. Maybe their son or daughter inherits Rachel's amazing singing talent and Finn's charm. They could struggle with living up to their parents' legacy in the glee club.
Definitely the children of Ladybug and Chat Noir. They would inherit the Miraculous and have big shoes to fill. Their personalities could be a mix of their parents' traits, like the daughter being as brave as Ladybug and having Chat Noir's sense of humor.
There could also be a character who is not directly related to the original ninja but has a special connection to the elemental powers. Maybe a young girl who discovers she has an ability to control an element that was previously thought to be lost. She would bring a fresh perspective to the group of the next generation and have to prove herself to the others.
One possible main character could be the son or daughter of Iron Man. They might inherit his genius and some of his tech - savvy ways. They could also have a bit of his cocky attitude at the start but grow into a true hero throughout the story.