There might be a new, mysterious enemy in the fanfiction. This enemy could be interested in the power that Naruto and his mother possess combined. They try to separate Naruto from his mother and then turn them against each other in order to obtain their power for some evil plan, like taking over the ninja world.
Perhaps a stray dog. The dog is always looking for something to chase or bother. When it sees the mommy cat taking care of the baby robin, it decides to make trouble. It tries to scare the cat away from the robin so that it can have fun chasing the small, helpless bird.
It could be a rogue ninja from another village who has a long - standing grudge against Naruto. Maybe this ninja has been plotting for years to take revenge on Naruto and when Naruto fakes his death, he starts to target Naruto's friends and family instead, thinking it will draw Naruto out.
It could be a new character, a rogue ninja from a long - lost clan. This antagonist might have a vendetta against Naruto because they see his potential to awaken the Rinnegan as a threat to their own plans for power. They could be trying to stop Naruto from achieving this new level of power so that they can carry out their evil deeds unopposed.
It could be Orochimaru. He's always been interested in Naruto's power. Maybe he devises a plan to make Naruto lose his memory so he can manipulate him more easily, perhaps to use him as a weapon against the Hidden Leaf.
Another possibility is that the main antagonist is a group. It could be a secret organization that Naruto discovers is plotting something big. Since they are so powerful and well - connected, Naruto fakes his death to infiltrate them and expose their plans from the inside.
It could be a relative from his family who was behind the abandonment. Maybe they want to keep Naruto away from some family secret or power, and they will do anything to stop him from finding out the truth.
Maybe it's an evil organization that wants to isolate Naruto. By making his family forget him, they hope to break Naruto's spirit and prevent him from becoming a powerful force in the ninja world. So they target his family's memories as a way to get to him.
It could be Kaguya. If Kaguya were to return and use her powerful abilities to drive Naruto mad, perhaps by invading his mind with dark thoughts and memories. She has the power to warp reality and minds, so she could easily make Naruto lose his sanity. And since she has a grudge against the world of ninjas, making Naruto, one of the most powerful ninjas, go insane would be a great way for her to start her reign of terror all over again.
Perhaps an old enemy from Naruto's past that was thought to be defeated has resurfaced. This enemy might have targeted Naruto's wife as a way to get revenge on Naruto for their previous defeats. They could be using her as a bait to lure Naruto into a trap.
A new and mysterious dark ninja could be the main antagonist. This character might have a special ability to corrupt others. They target Naruto specifically because of his power and influence. By using some form of dark jutsu or psychological manipulation, they make Naruto lose his humanity, turning him into a weapon against the very people he used to protect.