In this fanfiction plot, perhaps Thanos' actions in the Marvel universe start to affect Naruto's world. Naruto then sets out to find the source of the disturbance. When he encounters Thanos, he is initially outmatched. But as the story progresses, Naruto befriends some of the Marvel heroes who teach him about Thanos' powers. Naruto then trains hard, combines his ninja skills with new knowledge, and in a final epic battle, he manages to kill Thanos. The story could also include Naruto's internal struggle as he is in a completely different world with different rules and enemies.
The plot could start with Naruto somehow being transported to Aizen's world. He quickly realizes the danger Aizen poses. Naruto begins to train and adapt to the new environment, learning about reiatsu. As he battles minor foes related to Aizen, he gains knowledge about Aizen's weaknesses. Eventually, when facing Aizen, he uses his ninja skills and new - found understanding of this different power system to land a fatal blow.
In a Naruto kills Thanos fanfiction, Naruto's sealing techniques could play a major role. He could use sealing jutsu to seal away Thanos' powers or even Thanos himself. Naruto has experience in dealing with powerful foes and sealing them away. Also, Naruto has the ability to sense danger and react quickly. Thanos, on the other hand, may underestimate Naruto at first. By the time he realizes Naruto's true strength, it could be too late for him.
One possible plot twist could be that Peter uses his intelligence rather than just his powers to defeat Thanos. Maybe he discovers a weakness in Thanos' armor or a flaw in his plan that no one else has noticed. For example, he could find out that Thanos' Infinity Gauntlet has a time - limited recharge period for its powerful snaps, and he uses this knowledge to his advantage at a crucial moment.
In a 'Tony kills Thanos' fanfiction, the plot could start with Tony being haunted by his previous failures against Thanos. He decides to go on a solo mission to study Thanos more closely. He discovers a way to disrupt Thanos' connection to the Infinity Stones. With this knowledge, he devises a plan. He lures Thanos into a trap where he has set up a device that can nullify the power of the Stones. Then, in a tense battle, Tony uses his own advanced technology and combat skills to fight Thanos without the Stones' advantage and finally manages to kill him. This could be a story of redemption for Tony, showing his determination and ingenuity in the face of seemingly insurmountable odds.
One possible plot could be that Peter Parker discovers a new power within himself that gives him the edge to take on Thanos. Maybe it's a power related to the Time Stone that he manages to manipulate in a way that Thanos can't counter. He could use this power to outwit Thanos in a battle and finally deliver the killing blow.
Well, it could be that Naruto uses his shadow clone jutsu to overwhelm the Justice League members. Maybe he'd start with Batman, using his speed and clones to catch him off guard. Then for Superman, he might use some of his more powerful jutsu like the Rasengan, powered up with Sage Mode. But it's really a wild and crazy concept as the two universes are so different in power scales and nature.
In this fanfiction's plot, Naruto might team up with some of the local characters in the White Walker world. He could teach them some of his ninja skills in exchange for knowledge about the White Walkers. As the story progresses, Naruto discovers that there is a 'leader' among the White Walkers. He decides that to truly end the threat, he must take on this leader. He trains hard, combining his existing skills with new strategies he's learned from his new allies.
Yes, there are likely fanfictions out there where Tony kills Thanos. The Marvel universe has a huge fan base, and fans often create alternative storylines. Some might explore this scenario to give a different take on the Infinity War and Endgame stories.
One consequence could be that Peter Quill Parker becomes a hero. People all over the universe might look up to him. But he may also face some jealousy or envy from other powerful beings.
One possible plot could be that Hinata is somehow mind - controlled. Maybe an enemy uses a powerful genjutsu on her that makes her see Neji as a threat to Naruto or to the village. Under this influence, she attacks and kills Neji, which then leads to a great deal of guilt and self - loathing for Hinata once she breaks free from the control.