A mysterious dark sorcerer could be the antagonist. This sorcerer has his own reasons for not wanting Kushina and Naruto to be reunited. Maybe he wants to use Naruto's power for his own evil deeds and knows that Kushina will protect Naruto at all costs. So, he sends his minions to fight Kushina and tries to keep them apart through various magical means.
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.
Some rogue ninja from the Hidden Cloud Village might be antagonists. They could be after Naruto for his Nine - Tails power and see Kushina as an obstacle. They might set up traps and ambushes to prevent Kushina from getting to Naruto. Also, there could be a group of power - hungry shinobi within the Hidden Leaf Village itself who don't want Kushina to return as they fear it will change the power balance in the village.
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.
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.
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.
The plot could be that Kushina wants Naruto to reunite with his father. She could use her spiritual connection to Naruto to plant the idea in his mind. Naruto then embarks on a journey to find out more about his father, Minato. Along the way, he discovers secrets about the past, including how his parents really met and fell in love. This could also lead to him unlocking new powers related to his family lineage.
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.
It could be the village elders. They often have a lot of power and influence in the Naruto universe. If they believe Naruto is a potential threat or they have some ulterior motives, they could be the ones holding him back, thus acting as antagonists.
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 antagonists could be the original Naruto's friends and allies. Since this is an evil Naruto, people like Sasuke, Sakura, and Kakashi would likely oppose him and try to stop him from whatever evil plans he has in the past.