One great Harry Potter fanfiction about being alone could be those that focus on a character like Harry during his summers at the Dursleys. They might explore his feelings of isolation and how he copes with it. Maybe he discovers new magic during those alone times.
Some fanfictions might center around Luna Lovegood when she's alone in the Forbidden Forest. Since she has a unique connection to the magical creatures, when she's alone in that forest, she could be having secret interactions with them. These interactions could lead to new understandings of the magical world that she then shares with her friends at Hogwarts.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3). It has a large collection of Harry Potter fanfiction, and you can use the search function to look for stories with keywords like 'alone' or 'isolation'.
One great fanfiction could be where Harry feels alone during the Triwizard Tournament. Maybe he's isolated because of the jealousy and suspicion from other students. He might find solace in the Forbidden Forest, encountering some magical creatures that help him see his situation differently.
One popular 'Harry Potter left alone' fanfiction might be those where he has to fend for himself in the Muggle world without any wizarding help. For example, he could be exploring his magic in secret while dealing with the challenges of being an orphan in a non - magical environment.
One interesting story could be about Harry facing challenges alone in the Forbidden Forest. Maybe he discovers a hidden magical creature that no one else knew about and has to figure out how to deal with it on his own.
One example could be a fanfic where Harry uses his cunning to outwit Voldemort in a different way than in the original books. He might use his knowledge of Hogwarts' secret passages in a very clever way to set traps for the Death Eaters. Another could be a story where he manipulates the political situation in the wizarding world by being cunning and making alliances that no one expected.
I don't really encourage the idea of Hermione being tortured in fanfiction. It's a rather dark and disturbing concept. However, if you're looking for well - written Hermione - centric fanfictions that are more positive, there are many that focus on her intelligence and her adventures at Hogwarts without such dark themes.
One great fanfiction could be 'The Slytherin Bond'. In this story, Harry realizes that not all Slytherins are bad and forms a unique friendship with a group of them. They end up working together to solve mysteries within Hogwarts that the other houses are oblivious to.
One example could be that in certain fanfictions, Harry is able to tame a wild Thestral without much difficulty. Since Thestrals are usually only visible to those who have witnessed death and are quite difficult to approach, Harry's ability to quickly gain their trust shows his talent with animals. Another example might be that he can communicate with the magical creatures in the Forbidden Forest on a deeper level, getting them to help him in his adventures.
In the 'harry potter alone broken fanfic', it may explore a scenario where Harry is alone in a more metaphorical sense. He could be surrounded by people but still feel alone. The 'broken' aspect might be related to his relationship with magic. Maybe he has lost his faith in magic or his ability to perform it as he used to. This fanfic could be a deep dive into Harry's inner turmoil, his feelings of isolation and his struggle to regain his former self.
One great fanfiction could be 'The Lost Generation' which delves into the lives of the characters after the events of the books in a really interesting way. It gives a new perspective on how they deal with the aftermath of the war and their own personal growth.