One possible plot could be that an ancient evil is awakened and sets its sights on destroying Konoha. Naruto, as the Hokage or a respected ninja, rallies all his friends and allies at first. However, as the battle progresses, he realizes that the only way to truly stop this evil is by giving his own life. So he makes the ultimate sacrifice, using his chakra to seal the evil away forever, thus protecting Konoha at the cost of his life.
It could be a long - lost relative of one of the old villains like Orochimaru. Maybe this person has been in hiding, training and growing in power, and now wants to take revenge on Konoha for what happened to their family member.
One possible plot could be that Naruto and Haku are in a situation where an extremely powerful enemy attacks. Naruto, being the selfless hero, jumps in front of Haku to take a fatal blow. Haku is then left with the guilt and determination to carry on Naruto's will. She might vow to bring peace to the ninja world, just as Naruto had always dreamed. This could lead to Haku joining the Hidden Leaf and trying to bridge the gap between different ninja clans in Naruto's memory.
One possible plot development could be that Sakura is deeply affected by Naruto's sacrifice. She might vow to carry on his will and become even stronger. In the story, she could use Naruto's death as motivation to finally achieve peace in the ninja world. For example, she could take on Naruto's dream of uniting all the villages and work towards it with all her might.
One possible plot development could be that Naruto sacrifices himself to save Ino from a powerful enemy. This might lead to a great sense of guilt among the other characters, especially those in Naruto's circle. Ino herself might be forever changed, vowing to carry on Naruto's will in some way. She could become more determined to achieve peace in the ninja world, inspired by Naruto's selfless act.
Well, one possibility could be that Naruto sacrifices himself to save Fem Haku from a powerful enemy. Maybe it's during a battle against a new and dangerous foe that has emerged in the Naruto universe. His death would likely have a huge impact on the other characters, especially those close to him like Sasuke and Sakura. They might be filled with grief and a strong desire for revenge.
Another idea is that Naruto's death while protecting Sakura causes a rift between Sakura and Sasuke. Sasuke might blame Sakura for Naruto's death in some way, either directly or because he feels Naruto wouldn't have been in that situation if it wasn't for Sakura. This could lead to a complex and angsty storyline where Sakura has to deal with the guilt of Naruto's death as well as Sasuke's coldness towards her.
In this fanfiction, it could be that Naruto's sister is being targeted because she has information that could expose a powerful and corrupt group. Naruto, being the selfless hero he is, shields her from their attacks. His death becomes a symbol of resistance against this group, and his sister then takes on the task of bringing them to justice.
One way this could unfold is that Naruto and Tenten are on a mission together. They are suddenly attacked by a powerful and unexpected enemy. Naruto, being the selfless hero he is, sees the enemy about to strike a fatal blow on Tenten. He jumps in front of her without hesitation, taking the full brunt of the attack. His last moments could be him telling Tenten to keep fighting and protect the village.
One possible plot could be that Naruto is somehow transported to the Bleach universe. He meets Haku there, and they form a bond. When Haku is in danger from some powerful Bleach - related threats like Hollows or powerful Soul Reapers who misjudge Haku, Naruto steps in to protect him. In the process, Naruto, with his ninja skills but lacking the full understanding of the Bleach world's powers, gets overwhelmed and dies while trying to shield Haku.
One possible plot is that there is a breach between the Naruto universe and the Hellsing universe. This causes chaos as demons from Hellsing start to invade Konoha. Naruto, being the hero, tries to figure out how to stop them. Alucard arrives, not entirely as an ally at first. But as they both realize the common threat, they join forces. Naruto learns about the different nature of the enemies from Alucard and adapts his ninja skills accordingly.