Fanfic about Harry Potter coming back: What are some interesting plot ideas?

One idea could be that Harry comes back through a powerful ancient magic that was dormant. Maybe a hidden spell in the Forbidden Forest is activated by his friends' strong desire to have him back. Another could be that Death itself gives Harry a second chance because he has unfinished business, like fully ridding the wizarding world of the last remnants of Voldemort's dark magic.

One idea could be that Harry comes back with new magical powers. Since he has faced death, he might have a deeper understanding of magic that was previously unknown. For example, he could have the ability to communicate with the dead more freely, which would help him solve mysteries related to the wizarding world's past.

One idea could be that Harry travels back to the time when Voldemort was still at Hogwarts as Tom Riddle. He could try to expose Tom's true nature early on, preventing all the future disasters. Another plot could be Harry going back to the time when his parents were killed and using his knowledge of magic to save them, and then seeking revenge on those who supported Voldemort at that time.

Perhaps a fanfic where Lily Potter's spirit is able to communicate with Harry Potter during his difficult times at Hogwarts. She gives him advice and guidance that helps him through his struggles, like dealing with Snape's complex feelings towards him, or facing the challenges of the Triwizard Tournament. This would add an emotional and magical element to Harry's journey.

One idea could be Harry using his modern magic knowledge to shock the wizards of the past. For example, he might introduce spells that were lost over time.

One plot idea could be that Crookshanks discovers a hidden passage in Hogwarts that leads to an ancient wizarding library. He then leads Hermione to it, and they find a powerful spellbook that helps them in their fight against the dark forces. Another could be that Crookshanks has a secret ability to communicate with other magical creatures in the Forbidden Forest and he enlists their help to save Harry from a dangerous situation. Maybe he can talk to the centaurs and they come to the rescue when Harry is surrounded by Death Eaters.

One idea could be exploring Bellatrix's backstory before she became a Death Eater. Maybe she had a complex relationship with her family that led her down the dark path. Another could be a story where she has a change of heart, perhaps due to an unexpected encounter with a character like Harry Potter. For example, she could see some of her own past self in Harry and start to question her loyalty to Voldemort.

Maybe a fanfic where the goblins of Gringotts go on strike. Since Harry has some influence with them due to his own vault and his adventures there, he gets involved in the negotiations. Along the way, he uncovers a plot by some Death Eaters who are trying to take advantage of the chaos to steal valuable magical artifacts from the bank. It would be a mix of politics, mystery and action.

A plot could be that the Harry Potter headphones are a creation of a young, brilliant wizard who is trying to revolutionize the way magic and Muggle technology interact. The headphones start to cause chaos in Hogwarts as they interfere with the existing magic there. Some students love them and think they're the coolest thing ever, while the teachers are worried about the potential misuse. Meanwhile, there's a mystery about the true power of the headphones that the main character has to figure out before they fall into the wrong hands.

One idea could be Harry going back in time to his parents' era at Hogwarts. He could try to prevent certain events that led to their deaths. Another could be him going back to the time of the founders of Hogwarts and getting involved in the creation of the houses and Hogwarts' early rules.