A vengeful spirit that was trapped in a seal long ago. The OC Seal Master accidentally releases it while exploring an ancient ruin. The spirit blames all seal masters for its imprisonment and sets out to destroy the OC and any other seal masters it can find. It has unique powers related to seals that it uses against the OC, like being able to corrupt normal seals into harmful ones.
A powerful and rogue ninja who has also mastered some dark seal arts could be a great antagonist. He might be jealous of the OC Seal Master's natural talent and pure intentions with seals, and tries to steal their techniques or stop them from achieving their goals. For example, he could be creating cursed seals that are causing chaos in the ninja world.
An organization that believes in using seals for world domination. They see the OC Seal Master as a threat to their plans because the OC is trying to use seals for good and protect the balance in the ninja world. This organization could have its own set of highly skilled seal users who are constantly trying to outwit and defeat the OC.
In terms of character design, as a seal master, she would likely have a unique appearance. Maybe she has tattoos or markings related to sealing on her body. Her personality could be very studious and detail - oriented, given the complexity of seals. She would also need to be strong - willed, as dealing with the power of seals requires a great deal of self - control. Her presence in the fanfiction could attract other characters interested in seals, creating a community within the story.
Perhaps the OC Seal Master is an outcast in their village because of their unique seal powers. They then get involved in a plot where they have to prove their worth by using their skills to protect the village from a mysterious threat. For example, an unknown force is causing seals all over the village to malfunction and it's up to the OC to figure out what's going on and fix it using their in - depth understanding of seal mechanics.
A really interesting antagonist could be a mysterious figure from a hidden organization. This person has been secretly observing Naruto's family for a long time. They have a master plan that involves using Naruto's daughter as a key component. Maybe they want to use her unique chakra or abilities for some dark purpose. They would be constantly manipulating events behind the scenes, setting up traps and challenges for her, all while remaining in the shadows, which would make them a very menacing and unpredictable foe.
It could be a group of rogue ninja who are jealous of Naruto's new powers. They might band together to try and steal his knowledge of all ninja arts or defeat him to prove their own superiority.
Sasuke could still be an antagonist in a way. Maybe in this AU, he's more dismissive of the female Naruto because of his own ideas about strength and gender. He might see her as weak at first and that could create a lot of tension between them.
There could be a jealous fellow ninja from within the village who feels that Naruto is getting too much attention for his blade - mastery. This ninja could turn against Naruto and become an antagonist, perhaps even teaming up with external forces. Also, an ancient evil entity that was sealed away long ago might be awakened by the power of Naruto's blade and seek to destroy him and the village to regain its freedom.
I'm not sure specifically who Lemon is without reading the fanfiction. It could be an original character created by the author for this story, perhaps someone with a special connection to the seal master or with unique abilities related to the Naruto world.
It could be a new character created specifically for the fanfiction. This character could also be a seal master or have some special skills related to seals that directly oppose Naruto's. They might have a personal vendetta against Naruto or be from a rival village, and their encounters during the Chunin Exams would be filled with high - stakes battles and intense rivalry over seal - related matters.
A powerful sorcerer could be a great antagonist. He could be after the OC's unique power or trying to use the Power Rangers' powers for his own evil deeds. He would constantly create magical traps and illusions to deceive Tyler and the OC.
She could change the power balance. If she's a powerful seal master, she might be able to create seals that can counter powerful jutsus of the villains. For instance, if there's a new villain with a really strong attack jutsu, she could develop a seal to nullify it.