It could be a new character created just for the fanfiction. Maybe a rogue ninja who has mastered a dark and powerful jutsu that Naruto has no defense against. This antagonist could be seeking revenge on Konoha for some past wrong and sees Naruto as the biggest threat to his plans.
Perhaps in the fanfiction, Naruto is betrayed during the battle. By someone he trusted, like a comrade who was secretly working for the enemy. This betrayal catches him off - guard, and he is severely injured. Unable to fully recover, he succumbs to his wounds during the fight.
I'm not entirely sure which specific fanfiction this refers to. But if Naruto were to die and become like Kushina, it could be a very interesting concept. Maybe it involves some sort of reincarnation or magic. It could potentially explore Naruto's connection to his mother in a much deeper way, like inheriting her abilities or personality traits. However, without reading the actual fanfiction, it's hard to say exactly what would happen.
Fanfictions about Naruto being neglected and nearly dying can be quite intense. Some might depict him being pushed to the brink by the lack of support from the village. For example, he could be deprived of proper training or basic necessities. However, these situations usually lead to Naruto's character growth in a different way than in the original series. He might form unexpected alliances with outcasts or find new powers within himself.
Well, I'm not sure about a single highly recommended one. But in general, in these fanfictions, Naruto's death for Sakura can lead to many different plot developments. It could make Sakura stronger, or it could drive a wedge between her and other characters who might blame her in some way for Naruto's sacrifice.
If Hermione dies in the final battle in fanfiction, the story would change drastically. For one, Harry and Ron would lose their intellectual backbone. Hermione has always been the one with the plans and knowledge, so without her, they'd be floundering. The whole wizarding world would also feel the loss deeply as she was a key figure in the fight against evil. Her death would be a huge blow to the morale of the Order of the Phoenix and all those fighting against Voldemort.
Ron would be completely shattered. Harry was his best friend, and he would likely feel guilty for not being able to protect him. He might become reckless in his attempts to avenge Harry. Hermione would be grief - stricken but also determined. She would pour herself into finding a solution to the mess left behind. Ginny would be heartbroken. She might initially withdraw but then become a strong advocate for justice, using her own skills and magic to fight against Voldemort's regime.
One possible plot development could be that with Harry's death, Voldemort fully takes over the wizarding world. His reign of terror becomes absolute, and the remaining characters have to go into hiding or form a secret resistance. For example, Hermione might lead the resistance, using her intelligence to try and find a way to defeat Voldemort without Harry. Another aspect could be the emotional turmoil among the characters. Ron would be devastated, and his character could either sink into depression or be driven by a thirst for revenge. Ginny, too, would have to deal with the loss of her love and might become a more hardened and fierce fighter against the Dark forces.
He would be filled with rage. Naruto has a strong sense of justice and the fact that someone took Hinata away from him would fuel his anger. He would vow to find and punish whoever was responsible. He might even become reckless in his pursuit of vengeance, endangering himself and those around him. His friends would have to step in to try and bring him back to his senses.
For Harry, it would be devastating. Hermione was like a sister to him. He'd likely become more isolated and perhaps more determined than ever to defeat Voldemort, but also more reckless. Ron would be heartbroken. He had a special relationship with Hermione, and her death might make him feel like he failed her. Ginny would also be affected. She looked up to Hermione, and her death would be a shock to Ginny's sense of security in the group.