Well, Harry might face the challenge of getting access to restricted magic knowledge. As he is alone, he can't rely on Hermione's research skills. He may also have to deal with the fact that the world is turning against him, believing he is the cause of the chaos. Physical challenges like having to traverse dangerous terrains alone to reach the source of the evil are also possible. And he has to face the ultimate evil which has powers far beyond what he has ever faced before, without the support of his comrades.
Sure. Harry discovers a dark and powerful threat that endangers the entire world. Without relying on his friends, he delves deep into ancient magic and uses his own wits and courage. He faces numerous challenges and overcomes powerful dark forces. In the end, he manages to cast a powerful spell that nullifies the threat and saves the world.
One possible plotline could be that Voldemort realizes that Harry is the key to some greater power or knowledge that he wants to obtain. So, instead of killing Harry, he saves him from another dark wizard or a dangerous magical creature. Maybe he sees that Harry has a connection to an ancient magic that could help him rule the wizarding world in a more complete way.
Another challenge is that without a proper teacher, he may not fully understand the nuances of magic. Magic in the Harry Potter world has a lot of rules and ethics. Harry might accidentally break some of these rules while trying to teach himself. Also, he may not be aware of the proper pronunciation or wand movements for some spells, which could lead to ineffective casting.
In such a fanfic, one possible reason could be extreme depression. Maybe he faced overwhelming losses like the death of his closest friends and felt he couldn't go on. Another reason might be that he was under some kind of dark magic influence that made him lose his will to live.
One big challenge could be public opinion. The wizarding community has a long - held grudge against Death Eaters. So Harry would face a lot of criticism and distrust from the public. People might think he's gone to the dark side or that he's being naive.
A plotline could involve Petunia having a change of heart due to a magical event. For instance, she might accidentally stumble upon some of Harry's magic or receive a message from the wizarding world about his importance. This could lead her to stand up against Vernon and protect Harry. She could then start to teach Harry about his mother's side of the family and the magic that runs in his veins in a more positive way than the Dursleys ever did. And perhaps she even helps him develop his magic skills secretly before he goes to Hogwarts.
One possible plotline could be that Susan and Amelia Bones are in danger from dark wizards. Harry, using his skills and courage, battles the dark wizards and rescues them. After that, there could be a sub - plot about Susan being grateful and perhaps developing a friendship or a deeper connection with Harry.
One challenge could be finding a safe place to stay. The wizarding world is not that big, and there are many places that are easily detectable. They would need to use magic carefully to avoid leaving a trail for those who might be looking for them.
One challenge could be the unfamiliarity with the Fairy Tail world. Different rules of magic and different types of magical creatures might be new to Harry. Another is that the Fairy Tail characters themselves might be very powerful and have their own unique abilities that Harry has to figure out how to counter.
Another consequence could be that he has to face opponents in the tournament that are far more experienced or dangerous than he expected. In fanfic, the author might make these opponents even more formidable to test Harry in ways that the original tournament didn't. Also, his relationship with Cedric could be severely strained as they are now direct competitors, which could lead to some intense rivalries and emotional drama.