There could be an outside force or enemy that sees Naruto's potential and wants him to stay in the ANBU for their own devious purposes. This enemy could be secretly manipulating the village elders or the ANBU members to keep Naruto where he is. They might even try to frame Kushina as a traitor if she gets too close to getting Naruto back.
Some members of the ANBU themselves could be antagonists. They might think that Naruto belongs with them and that Kushina is interfering in their affairs. They could try to keep Naruto in the ANBU by any means necessary, including using force or setting up traps for Kushina when she tries to approach Naruto.
The Akatsuki could be antagonists. They might see Kushina's return as a threat to their plans. Since Kushina is a powerful Uzumaki, her presence could disrupt their schemes related to the tailed beasts. They could try to stop her from reaching Naruto or even try to capture her for their own purposes.
One possible plot development could be that Kushina uses her influence within the village to start a formal request for Naruto to be returned from the ANBU. She might gather support from other characters who also care about Naruto's well - being, like Iruka or Jiraiya. This could lead to a series of political discussions within the village leadership about the role of the ANBU and whether Naruto's skills are better utilized elsewhere.
Maybe a new group of rogue ninjas who see Naruto as a threat to their plans. They've kidnapped him and are holding him captive. Anko would then have to fight these ninjas who are well - trained and have their own unique abilities. They might be after Naruto's Nine - Tails chakra for some evil purpose.
The other gods might be antagonists. For example, Zeus could be against Artemis getting Percy back because he doesn't trust Percy or he has his own plans for Percy that don't align with Artemis'. Another god like Ares could also be an antagonist as he might see Percy as a threat and not want him to be on Artemis' side.
Another aspect could be the emotional impact on Naruto. Having his mother back would be a huge boost to his mental state. He's always longed for family, and Kushina's return would fill that void. It could change his perspective on life and make him even more determined to protect the village and his loved ones.
In such a fanfic, perhaps Kushina manages to break free from the afterlife's constraints. She could use some sort of ancient jutsu that allows her to return to the living world. Then, she starts her journey to find Naruto. Maybe she disguises herself as an ordinary traveler at first to observe Naruto from afar, seeing how he has grown and all the hardships he has faced. Once she reveals herself, Naruto is initially shocked but then filled with joy at the thought of having his mother back.
Kushina could also seek the help of other powerful shinobi. For instance, she might ask Minato for assistance. Since Minato was Naruto's father and also a very skilled ninja, together they could come up with a plan to reunite with Naruto. Maybe they would use some sort of space - time ninjutsu to get closer to him.
One possible plot development could be that Kushina's spirit somehow manages to break through the boundaries between the afterlife and the living world. She then tries to communicate with Naruto, perhaps through dreams at first. This could lead to Naruto starting to search for a way to bring her back permanently, involving him in some sort of ancient and dangerous ritual that he discovers in some long - forgotten scroll.
The main antagonist could be the evil Naruto himself. Since he has turned to the dark side, his actions and motives would be against the well - being of the village and those around him, including Kushina. He might be the source of conflict and chaos.
Maybe an ancient evil that has been awakened. This entity is so powerful that it takes the combined godlike powers of Naruto and Kushina to stand against it. The antagonist could have some connection to the origin of chakra or an old, forgotten curse.