It could be someone who is jealous of their new - found bond. Maybe a rival of Uzumaki who doesn't want him to have such a powerful ally in his sister.
There might be a group or an organization that has some hidden agenda related to their family. They see the twins' reunion as a threat to their plans and try to keep them apart or harm them. For instance, if their family has some special power or knowledge, this antagonist group wants to control or destroy it.
It could be someone who doesn't want Naruto and his sister to unite. Maybe a rogue ninja who fears their combined power. He might try to drive a wedge between them by spreading false rumors or causing misunderstandings.
Well, in such a fanfiction, they might initially be shocked to discover each other. Maybe they start to share their different life experiences growing up. For example, Uzumaki could talk about his adventures and training, while his sister could share about her own challenges in a different environment.
It could be a rogue ninja from a forgotten clan who has a vendetta against the Uzumaki. This antagonist might see Shinachiku as a threat because of his potential power and decides to target him and Boruto to stop them from achieving something great in the Hidden Leaf Village.
It could be an ancient monster that has a grudge against the family of Poseidon. Since Percy and his twin are children of Poseidon, this monster might target them specifically. Maybe it's a sea serpent that was once defeated by Poseidon and now wants revenge by going after his descendants.
One possible main antagonist could be a powerful ghost who sees the combination of Danny and his twin sister's powers as a threat. This ghost could be trying to split them up or turn them against each other so that they can't use their powers together to stop his evil plans. Maybe he has some connection to their family's past in the Ghost Zone that makes him particularly interested in them.
It could be Frieza. Since Frieza has a history of hating the Saiyans, he might see Goku's sister as a new threat and try to eliminate her.
Maybe there's an evil spirit that has a grudge against the dragons. When the sister starts to get involved with the dragons, this spirit targets her. It could be a spirit that was once wronged by the dragons in the past and now wants revenge, not caring that the sister is just a new acquaintance to the dragons.
In a 'naruto meets his twin sister fanfiction', perhaps his sister has been living in a different village all this time. When they meet, she could be a very powerful ninja in her own right. Their meeting could stir up a lot of emotions, like Naruto feeling a sense of family he's never had before. They might team up to face a common enemy, using their shared bloodline abilities in unique ways.
It could be an unknown relative of Naruto. Maybe someone who has been living in a far - off place and just discovers their Uzumaki heritage.
It could be Orochimaru. He's always been interested in powerful bloodlines, and the Uzumaki's power would be no exception. He might try to do something to Naruto that would piss off an Uzumaki, like trying to steal his chakra or use him for some evil experiment.