Another possible antagonist could be a new, powerful enemy from outside the village. This enemy might be after Naruto because of his status as a jinchuriki and now, with Kurenai as his sister, they could also target her. This new enemy could have unique and dangerous abilities that Kurenai and Naruto have to figure out how to defeat together.
The Akatsuki could be the main antagonists. Since Naruto is an important target for them, and if Kurenai is now his sister, they might see her as an obstacle in their way. They could try to use their various powers and strategies to either capture Naruto or cause harm to both him and Kurenai.
It could be some rogue ninja from within the Hidden Leaf Village who oppose the idea of Kurenai being Naruto's sister. They might fear that this new connection could disrupt the power balance in the village. These rogue ninja could form a secret group and plot against Kurenai and Naruto, trying to drive a wedge between them and the rest of the village.
It could be some of the historical villains from Naruto's past that were thought to be long gone but are now resurfacing due to the time travel. For example, Madara Uchiha might be an antagonist if they travel back to a time when he was still active and plotting against Konoha.
The Sound Ninja could be the main antagonists. They might be after some valuable information or technique that Kurenai's team has. Their aggressive and unethical ninja ways would contrast with Naruto and his team's values, leading to intense battles and a fight for what's right.
It could be some rogue ninjas who are after Lemon for some reason. Maybe they think she has a special power that they can use for their evil plans.
The main antagonists could be the traditional Naruto villains like Orochimaru or the Akatsuki members. They might target Naruto's sister as a way to get to Naruto, seeing her as a weakness or a way to manipulate him.
Well, it could be about Kurenai having some secret connection to Naruto as his sister. Maybe she discovers this truth and has to deal with the implications. For example, she might have to protect Naruto from new threats while also coming to terms with her new - found family relation.
The Foot Clan could be a main antagonist. They might see the baby sister as a way to get to the turtles, either by kidnapping her or using her to blackmail them. Another could be Shredder himself if he returns. He would surely not pass up the opportunity to use the baby sister against the turtles. And Baxter Stockman could also be an antagonist. He might be interested in using the baby sister for some of his mad - scientist experiments.
One possible antagonist could be a jealous rival among the pirate gangs. This person might envy Harry Hook's sister's skills or popularity. Additionally, a corrupt official in the Isle of the Lost who wants to keep the status quo and sees her as a troublemaker could be an antagonist. Also, if there are any ancient curses or dark spirits on the isle, they could turn against her and try to stop her from achieving her goals.
Well, it could be about Kurenai taking on a sisterly role for Naruto. Maybe she helps him train, like teaching him some special genjutsu techniques that she's good at. She could also be there to support him emotionally, especially when he faces difficulties in the village or in his ninja missions.
The Sound Ninjas could be great antagonists. They could use their own unique parkour - like skills to try and stop Naruto. They might be chasing him through the forest, and Naruto has to use his parkour skills to outmaneuver them.
Maybe some corrupt officials within the prison system itself. They oppose Naruto's more humane and reform - focused approach to prison management. They might try to undermine him at every turn, for example, by cutting off resources or spreading false rumors about him among the prisoners.