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 Akatsuki could be the antagonists. Even though they were mostly defeated in the original story, in this fanfiction, some remnants of the Akatsuki could be after Naruto again. They might think that Naruto still holds some valuable power or information that they can use. Anko would have to go against these powerful and dangerous foes to bring 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.
Well, perhaps in this fanfiction, Naruto has been kidnapped by an unknown enemy. Anko, having a soft spot for Naruto, uses her skills and connections to track him down. She might have to face off against other powerful foes who are also after Naruto for their own reasons. The story could involve her teaming up with some of Naruto's old friends, like Shikamaru or Sakura, to rescue him and bring him back to Konoha.
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.
Perhaps a rogue ninja group that has a vendetta against Anko. When she forms this new family with Naruto, they target the whole family unit to get back at her. They might see Naruto as a way to get to Anko or vice versa, and constantly attack and create chaos for them.
Anko might not be a traditional antagonist, but if she is siding with Evil Naruto for some reason, she could be seen as an antagonist from the perspective of the other villagers. However, it's also possible that she is being manipulated herself, in which case the true antagonists would be those doing the manipulating.
Well, in fanfiction, Naruto might want to marry Anko because she represents a different kind of person compared to the other girls in his life. She's older and has a more mysterious air. The writer could develop a story where Naruto, as he grows up, realizes that Anko is the one who truly understands him on a deeper level. And so, his desire to marry her comes from that deep connection.
The antagonist could be a new, self - proclaimed ninja god who wants to rule over all the villages. He sees Naruto as the only threat to his power, so he used a powerful spell to take away Naruto's godlike abilities. Konoha wants Naruto back to stop this tyrant. This antagonist might have an army of brainwashed ninja at his command.
It could be a new group of rogue ninjas who have a vendetta against Konoha. They might be led by a powerful and cunning ninja who has a personal grudge against Naruto or the village in general.
It could be the hospital's administration. They might oppose the idea of Cameron getting pregnant, either because they think it'll affect her work performance or because they don't want any drama in the workplace.