It could be a powerful ninja - demon hybrid. This antagonist could be jealous of the bond between Naruto and Inuyasha. He wants to use their combined power for his own evil purposes. So, he tries to turn them against each other first by spreading false rumors and then by attacking their loved ones.
It could be a powerful ninja who wants to capture the fox brother for his own purposes. Maybe he believes that the fox brother's powers can be used to take over the ninja world. This antagonist might be from a rival village or a secret organization within Naruto's own village.
An ancient dark magic entity that has a vendetta against the Potter bloodline. It targets Harry and James, and they have to uncover the secrets of their family's past to defeat it. This antagonist could bring in a lot of mystery and exploration of the Potter family history.
There could be a dark spirit that has been awakened by the little brother's emerging powers. This spirit is angry at being disturbed and decides to target the little brother. It causes chaos in Arendelle, blaming it all on the little brother. As the story progresses, the little brother has to find a way to either defeat or pacify this dark spirit to save his home and family.
A powerful sorcerer who wants to steal the brother's space powers for himself could be the main antagonist. He could be an ancient being who has been waiting for the right moment to take over the demigod world, and he sees the brother's powers as the key to achieving his goal.
It could be the other students at Hogwarts who get fed up with the brother's idiocy. They might start to bully him or play mean pranks on him, which then forces Harry to step in and protect his brother. So in a way, the group of mean - spirited students become the antagonists.
It could be another powerful demon who sees the twin brother as a threat. Since he's new to the demon world, this more experienced demon might want to eliminate him.