Maybe a new villain could be introduced. A powerful ninja from a long - lost clan who sees Naruto's new power as a threat to their own plans of world domination. This antagonist could have unique powers that are specifically designed to counter Naruto's dual - Jinchuriki abilities.
It could be Orochimaru. He's always been after power and Naruto having the power of two Bijuus would be very tempting for him. He might try to extract the Bijuus from Naruto to use for his own evil purposes.
Another possibility is that the Akatsuki, even though they were mostly dealt with in the original series, could reform with a new leader who is hell - bent on getting Naruto's two Bijuus. They could see it as the key to finally achieving their goals of controlling all the Bijuus and thus the ninja world.
Well, it could be about Naruto having to deal with the power of two Bijuus. Maybe he struggles to control their combined power at first. He might face new and more powerful enemies who are after the power he now holds. And there could be a whole new arc of training to master this unique situation.
It could be Orochimaru. He's always after power and seeing Naruto with Son Goku's power would make him a prime target. He might try to extract the power from Naruto for his own evil experiments.
In this fanfiction concept, Naruto's new abilities could be vast. He might be able to access a new form that combines the features of all the Bijuus he houses. This form could have multiple tails, each representing a different Bijuu, and with each tail comes a different power. For instance, one tail could give him the ability to control water like the Three - Tails, while another could grant him the power of fire like some of the other Bijuus. He could also develop the ability to share the power of the Bijuus with his allies in a limited way, making his team even more formidable in battle.
Well, it could change the plot in many ways. For one, Naruto would be incredibly powerful right from the start. His relationships with other characters might also be different. Since he's the Jinchuriki of multiple Bijuus, the village's view of him could be more of a weapon than a person. And he might have to deal with more internal struggles as he tries to control all that power.
There could be a group of anti - ninja revolutionaries as the main antagonists. They see Naruto's return with the swords as a threat to their plan of overthrowing the existing ninja order. They try to take the swords from Naruto to use them as symbols of their new regime, believing that the power of the swords will help them convince other ninjas to join their cause.
It could be an evil ninja from Naruto's world who has somehow also gained access to the powers of the Beyond Two Souls world. This character could use these new powers to try and take over both worlds.
Often, other Naruto characters like Naruto Uzumaki or Kakashi Hatake might be included in the two - tails jinchuriki fanfiction. Their interactions with the two - tails jinchuriki can create new and exciting plotlines, whether it's them helping the jinchuriki control their power or going on adventures together.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
Maybe a new character could be introduced as the main antagonist. This character could be from a hidden village that has been secretly observing Naruto's growth. They realize his potential as a weapon and plan to steal his power or use him to start a war against other villages.
Krampus himself could be the main antagonist. Since he is a figure known for punishing the naughty, his actions in the Naruto world might be seen as unjust. He could be going after ninjas without really understanding their motives, causing a lot of trouble for the main characters.