Perhaps the writer of the fanfiction is trying to create a different character arc for Harry. Maybe he has faced so many hardships that he becomes disillusioned. For example, if he has lost too many loved ones and feels that his efforts are in vain, he might stop trying. It's a way to add a new layer of depth to his character.
The plot takes a darker turn. Without Harry's determination, the fight against evil might seem almost lost. His friends would have to scramble to fill the void he leaves. For example, Hermione might become the main driving force, but she would face many challenges without Harry's leadership.
The concept of 'Harry Potter stops Dumbledore' might also be related to character development. Harry has always respected Dumbledore, but as he matures, he may start to question Dumbledore's actions. By stopping Dumbledore, he is asserting his own beliefs and values. It could be in a situation where Dumbledore's actions were endangering the school or their friends. Harry, having a different perspective from his own experiences, decides to intervene to protect what he holds dear.
Well, perhaps Harry could use his magic skills like the Patronus charm to hold Loki at bay. Since Harry has experience dealing with dark forces, he might be able to outwit Loki's trickery.
The plot could start with Thanos arriving in the wizarding world. Harry senses the great danger he poses. Harry would first try to understand Thanos' powers and motives. He might gather information from the Ministry of Magic's records or from ancient magical texts. Then, he would start to form a team of wizards and witches who are willing to fight. As the story progresses, they would engage in battles with Thanos and his minions. Harry would use his Patronus charm to protect his team from Thanos' dark energy, and he would gradually figure out a way to stop Thanos, perhaps by using a combination of powerful spells and the unique magical environment of the wizarding world.
Maybe Harry Potter could stop Thanos with the help of his friends. Hermione's intelligence could come up with a plan. They could use time - turners to go back in time and prevent Thanos from getting the Infinity Stones in the first place. Ron could be there for moral support and also use his chess - playing skills to outwit Thanos in some strategic battles.
Loki is a powerful Asgardian with abilities that are more in the realm of the supernatural in a different way than Harry's magic. But Harry has a strong will and has faced some extremely tough opponents. In a fanfiction, if the writer decides to give Harry access to some of the most powerful and ancient magic in the wizarding world, he could be more powerful. For example, if he could use the magic that created the Hogwarts itself, he might be able to stop Loki quite effectively. Also, Harry has the support of his friends and the entire wizarding community behind him, which could give him an edge in power.
One possible reason could be that he has been through so much in his original adventures that he's just tired. He might feel like he's constantly being put in new, often contrived, situations in the fanfiction and he simply wants to step away from all the chaos and drama.
The Avengers would be shocked at first. They are not used to magic and seeing Harry Potter just swoop in and stop the Snap would be quite a surprise. But then they would be extremely grateful. They might start asking Harry a million questions about his magic and how he did it.
If Harry Potter stops holding back in fanfiction, he could become an even more powerful and assertive character. He might use his magic more freely and without hesitation, taking on greater challenges and enemies. This could lead to some really exciting and action - packed storylines, with Harry showing off his full potential.