Perhaps a new god created from the combined dark magic of the slain Norse gods. This new entity would be full of rage and a desire for revenge against Kratos for decimating their kind. It could have powers that are a mix of all the gods it was created from, making it an extremely difficult foe to defeat.
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.
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.
The fanfiction might explore Atreus' journey on his own. He could travel to different realms in search of a way to bring back his mother. Along the way, he meets new characters, perhaps a powerful sorceress who tries to use him for her own gain, but he outsmarts her and continues his quest.
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.
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.
The main character could be a person who was once a part of an evil organization but has had a change of heart. In the 'pokemon next generation fanfiction', they use their knowledge of the organization's ways to protect the new generation of Pokemon. They start with a single Pokemon that they rescued from the organization and build a team as they go, facing challenges from their former comrades and also making new friends among other trainers.
The proteges of the Avengers could be main characters too. Maybe Hawkeye has been training a young archer who has amazing accuracy but lacks confidence. This protege could be part of the next - generation Avengers and grow throughout the fanfiction, facing challenges that force them to become more self - assured.
New students from different kingdoms could also be main characters. For instance, a student from Vacuo who has a unique sand - based fighting style. Or a student from Atlas who is a tech genius and uses advanced weapons. They could team up with the descendants of Team RWBY to form a new, diverse team.