One idea could be that Neville Longbottom is secretly a powerful wizard all along but has been suppressing his power due to self - doubt. Another could be that the Hogwarts portraits have their own little society and gossip network within the castle walls. And a third one might be that Hagrid has a secret family of magical creatures in the Forbidden Forest that he takes care of.
It can add more depth to minor characters. For example, by creating headcanons about characters like Lavender Brown, we get to see a side of the Hogwarts student body that wasn't fully explored in the original. It gives them more personality and backstory.
One interesting headcanon could be about the backstories of the Servants. For example, there might be a headcanon that a certain Servant had a secret friendship in their past life that influenced their character in the game. Another could be related to the magic system in the Fate Grand Order universe, like a unique way of casting spells that isn't shown in the main story.
One sad Harry Potter fanfiction could be about the death of a beloved character like Sirius Black, exploring Harry's grief in more depth than in the original books. Another might be set in an alternate universe where Harry fails to defeat Voldemort and has to live in a world full of despair. There could also be a fanfiction focusing on Harry's lonely childhood before Hogwarts, delving into his feelings of isolation.
One sad story is the death of Sirius Black. He was like a father figure to Harry, and his sudden death in the Department of Mysteries was tragic. Harry had to watch him get hit by Bellatrix's spell and disappear through the veil. It was a moment of great loss for Harry, who had finally found a family member in Sirius.
Betrayal is a big theme. When Harry is betrayed by someone he loves, such as a teacher or a fellow student, it causes him great sadness. It breaks his trust and makes him question everything. Also, the theme of unrequited love can be seen. If Harry has feelings for someone who doesn't return them, it adds to his sadness. In some fanfictions, his love for Ginny might be unreciprocated, leading to his heartbreak. Additionally, the theme of feeling like an outcast, not fully belonging in either the Muggle or wizarding world, can make him sad.
One sad Harry Potter fanfiction is 'The Lost Years'. It delves into Harry's post - Hogwarts life filled with loneliness and the memories of those he has lost. Another could be 'A Broken Bond' which focuses on the fractured friendship between Harry and Ron due to a misunderstanding. There's also 'Silent Screams in Hogwarts' that shows the hidden pain of the students during the dark times at the school.
One sad story could be about a character who was left behind after the Battle of Hogwarts. They might have lost all their friends and be struggling to find a purpose in life. They wander the now - peaceful halls of Hogwarts, haunted by memories of the war and those they've lost.
You can try fanfiction websites like Archive of Our Own (AO3). There are tons of 'Harry Potter' fanfics there, and you can easily search for sad ones by using relevant tags like'sad' or 'angst'. Another option is Fanfiction.net, which also has a large collection of 'Harry Potter' fan - created stories.
There are fanfics that focus on Snape's unrequited love for Lily in a very sad way. For example, some might explore his internal thoughts and feelings more deeply, showing his pain and longing as he watches Harry, a constant reminder of what he lost. And then there are those that deal with the death of Sirius in a more emotional and in - depth way, like how it affects Harry not just immediately but also in the long run, his trust issues getting worse.