Perhaps an old enemy of Itachi's from his past who has also reincarnated or found a way to exist in the modern day. This antagonist would try to use Naruto's kids as a way to get revenge on Itachi, seeing them as a weak point in Itachi's new - found role as a protector.
The plot might involve Itachi taking Naruto's kids on adventures around the modern city. They could visit places like museums or amusement parks, and along the way, Itachi would teach them important life lessons. He might also introduce them to aspects of his own culture that are relevant in the modern world.
In a 'nitachi takes naruto's kids modern day au fanfic', the plot could be centered around Itachi trying to give Naruto's kids a different perspective on life. Since he has a complex past, he could use his experiences to teach them about making the right choices. Itachi might also be involved in protecting the kids from some modern - day threats that are related to the remnants of the old ninja clans. For example, there could be a group that wants to use Naruto's kids for their own evil purposes, and Itachi has to stop them while also teaching the kids about their own heritage and powers in the context of the modern world.
Well, to find a 'nitachi takes naruto's kids modern day au fanfic', first you need to be clear about what kind of plot you expect in it. Is it about Itachi becoming a mentor to Naruto's kids? Or something more like a family - bonding kind of story? Once you have a better idea, it'll be easier to search for. You can also join Naruto - themed fan communities and ask for recommendations there.
It could be Orochimaru. Since he's always after power and Naruto, with Itachi's training, would become a more appealing target. Orochimaru might see Naruto as a threat to his plans of obtaining all the jutsu in the ninja world.
It could be the Akatsuki. Since Naruto is now following a more Sasuke - like path, he might attract their attention more. They might see him as a potential threat or someone they can manipulate, just like they did with Sasuke.
Itachi himself could be seen as the main antagonist in this scenario. After all, he is the one doing the kidnapping.
It could be Orochimaru. Since he is always scheming and has a lot of power, Naruto could be reliving a day where Orochimaru is about to launch a major attack on the village. Each day, Naruto has to figure out how to stop Orochimaru's different plans, whether it's his experiments, his infiltration attempts, or his attempts to steal jutsu.
It could also be Lila. In this au, she could be a master manipulator who uses her lies to create chaos among the heroes and the citizens. She teams up with other criminals and uses her charm to get away with various crimes while making Ladybug and Chat Noir look bad. She might even be able to get her hands on some sort of powerful weapon or item that gives her an edge over the heroes.
Another possibility is a new character created specifically for the fanfic. This antagonist could be a dark - chakra user who believes that the Samaheada's power should be used to reshape the ninja world according to his own twisted vision. He would view Naruto as a threat not only because he wields the Samaheada but also because of Naruto's reputation for bringing hope and change to the village.
It could be Orochimaru. He's always been interested in unique powers and abilities. The idea of X - 23's mutant abilities and Naruto's jinchuriki powers would be very appealing to him. He might try to capture them both to use in his experiments to gain ultimate power.