Another possibility is a spirit that was wronged by Kagome's family in the past. When Kagome meets her father, it senses their reunion and decides to cause trouble. It might try to possess one of them or create chaos in their village to get back at them for what happened in the past.
It could be a demon who has a grudge against Kagome's father. Maybe her father defeated this demon long ago, and now that Kagome is in the picture, the demon sees it as a chance to get revenge on the family.
Well, it could be about Kagome having an emotional reunion with her father. Maybe she discovers things about her family's past through him, like some hidden powers or secrets that have been passed down. And they might go on an adventure together to protect their family or the village.
It could be Naraku. Even though the focus is on Kagome and child Bankotsu, Naraku is always lurking in the shadows in the Inuyasha universe. He might see Kagome's interaction with Bankotsu as a threat to his plans and try to manipulate the situation to his advantage. He could try to turn Bankotsu against Kagome or use Kagome's distraction with Bankotsu to carry out some other evil deeds.
If the fanfiction is set in Columbia from Bioshock Infinite, Comstock would likely be the main antagonist. His fanatical religious beliefs and his plans for Columbia would clash with Kagome. Comstock wants to maintain his power and carry out his dark plans, and Kagome's presence, with her different way of thinking and her powers, would disrupt his carefully laid - out schemes.
It could be a dark wizard who is trying to prevent Teddy from meeting his father. This wizard might be afraid that Teddy will gain some powerful knowledge or magic from his father that could be used against him.
It could be a powerful dark spirit from Kagome's world that somehow manages to cross over to the Naruto universe and starts causing chaos. This spirit might be immune to normal ninja attacks and require Kagome's spiritual powers to be defeated.
A new character could be created as the main villain. For example, a corrupted spirit from Kagome's world that has the ability to possess the Transformers. This entity could use the Transformers' strength and technology for its own evil purposes, causing chaos in both worlds. It would be a formidable foe as it combines the magic of Kagome's world with the mechanical power of the Transformers.
Maybe a demon that has somehow managed to cross over into Kagome's time without being fully noticed at first. This demon could be after Shippo for some reason, perhaps because of his connection to Inuyasha or his own unique fox abilities. As Shippo and Kagome start to figure out what's going on, the demon becomes the main source of conflict in the story.
It could be a powerful warlock who has found a way to manipulate time and space. He wants to stop Paige and Prue from joining forces because he fears their combined power. So he creates various obstacles and illusions to keep them apart.
Maybe a powerful rogue ninja who has discovered a way to steal the powers of divine entities. He sets his sights on Amaterasu and Naruto because he knows that if he can combine their powers, he will be unstoppable. He creates various traps and illusions to separate them and weaken them so that he can carry out his evil plan.
A powerful new pirate group could be the main antagonist. They might be after the same treasure or territory as Luffy and Shanks. They could have some unique and dangerous abilities, like the ability to control sea monsters. Their leader could be a former crewmate of a well - known and feared pirate, who has his own vendetta against both Luffy and Shanks for some past reasons.