The Akatsuki could be the main antagonists. They are always after the tailed beasts, and if they find out that Ino has some sort of access to the Kyuubi, they would surely try to capture her and use her to get to the Kyuubi. They could pose a serious threat to Ino and Naruto throughout the fanfiction.
Another possibility is a rogue ninja who has a vendetta against the Hidden Leaf. This ninja discovers Ino's new - found connection with the Kyuubi and decides to use it to destroy the village. He could try to manipulate Ino or even kidnap her to force the Kyuubi to do his bidding, making him a formidable antagonist in the story.
It could be Orochimaru. He might want to use Ino's connection with the Kyuubi to his own advantage. Maybe he plans to extract the Kyuubi's power through Ino's mental link with it.
One possible plot could be that Ino, with her mind - related abilities, somehow manages to make a connection with the Kyuubi's chakra within Naruto. Maybe she's trying to understand the nature of the Kyuubi's power for some research. This could lead to a series of events where she starts to see memories or emotions associated with the Kyuubi that no one else has been able to access before.
It could be an evil ninja who wants to capture the fem Kyuubi for her power. He might see the unique bond between Naruto and the fem Kyuubi as a threat to his plans of world domination.
It could be Orochimaru. He might be interested in Ino's mind - control abilities and Naruto's Nine - Tails chakra for his own evil experiments. He could try to capture them both and use their powers to create a new type of jutsu or even a new, more powerful being under his control.
There could be an internal antagonist within Konoha as well. Maybe a jealous ninja who has always had a crush on Shikamaru or envied Ino's skills. This person could spread false rumors about Ino's pregnancy, causing unnecessary stress and drama for the couple. They might try to drive a wedge between Shikamaru and Ino during this vulnerable time.
It could be an ancient shinobi who once tried to control the Kyuubi but was sealed away. Now, with Naruto as the new host, he sees an opportunity to take over the Kyuubi's power again.
It could be Orochimaru. He's always been interested in power and the secrets of the Kyuubi. With Naruto releasing the Kyuubi fuuinjutsu, Orochimaru might see this as an opportunity to either steal Naruto's power or use him for his own evil experiments.
Obviously, the fem Kyuubi herself could be the main antagonist. Since she is the one manipulating Naruto, her actions are causing problems and driving the conflict in the story.
Madara Uchiha could also be a main antagonist in this fanfiction. He has grand plans for the ninja world and the released Kyuubi could be a key part of his scheme. Since Naruto is dead and can't oppose him, he might try to use the Kyuubi to reshape the world according to his vision. He has the power and the ambition to be a major threat in this situation.
A possible main antagonist could be a powerful dark wizard who has somehow learned about the existence of the demigod world and wants to harness the power of the demigods for his own evil plans. He could be jealous of the unique abilities that Percy and Annabeth possess and see them as a threat to his rule in the wizarding world.
It could be the Red Queen. She might see Alice's presence in Mirana's world as a threat to her own power and try to stop their friendship.