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.
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.
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.
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.
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.
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.
Romance - based fanfictions are quite popular. For example, pairings like Naruto and Hinata are common. Many fans enjoy reading about their relationship development.
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.
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.
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.
There are various scenarios in Naruto love Anko fanfiction. One could be where Naruto helps Anko deal with her past traumas. His positive attitude might start to chip away at Anko's tough exterior. Maybe they start off as friends, training together, and over time, a deeper connection forms. Another possibility is that they are thrown into a dangerous mission together and their feelings for each other grow as they rely on one another for survival.