One idea could be that in the alternate dimension, Dementors are actually friendly creatures that help protect Hogwarts instead of causing fear. They could use their powers to keep dark wizards at bay in a different way.
In the 'Harry Potter alternate demention fanfic', it could be that the Dementors exist in a world where Voldemort never rose to power. Without the threat of dark wizards like him, the Dementors have evolved into a different kind of being. They might be more like spiritual guides, leading lost souls to a place of peace rather than sucking out their happiness. This would completely change the role of the Dementors in the story and open up a whole new set of adventures for Harry and his friends.
Maybe in this alternate dimension, the Dementors are the result of a failed experiment to create a new form of Patronus. So they have some strange abilities related to positive magic as well as their usual negative ones. This could lead to a very different dynamic within the wizarding world. For example, Harry might have to learn how to control these 'new' Dementors instead of just fending them off.
In an alternate dimension, the main antagonist could be a dark entity that has emerged from the very fabric of the dementions themselves. This entity could be using the Dementors as its minions, controlling them to do its bidding. It would be a powerful and mysterious foe, and Harry would have to discover its true nature and weaknesses in order to defeat it.
One idea could be that in the alternate dimension, Voldemort won the war long ago. Hogwarts is now a dark and dreary place, with Death Eaters as teachers. Harry and his friends, who are part of an underground resistance, have to find a way to travel back in time to change the course of history.
One idea could be that in this alternate universe, Hufflepuff is the dominant house in Hogwarts. They have a secret society within the house that holds the key to an ancient magic that can save the wizarding world from a new, unknown threat. And the main character, a Hufflepuff student, discovers this by accident and has to take on the responsibility to gather the members of the society.
One interesting idea could be that Harry decides not to defeat Voldemort in a traditional battle. Instead, he uses a powerful spell of love and redemption to turn Voldemort back to his original self, Tom Riddle, a lost and confused boy. This would completely change the whole dark - vs - light dynamic of the story.
One idea could be that Harry has an alternate identity as a Muggle detective. He uses his magical skills secretly to solve cases that have a magical element but are disguised in the Muggle world. For example, he might be investigating a series of strange disappearances that are actually the work of a dark wizard trying to gather ingredients for a powerful spell.
One interesting plot could be if Harry was sorted into Slytherin instead of Gryffindor. It would completely change his relationships and how he's perceived. He might have to deal with the prejudices of his own house while also trying to figure out his role in the fight against Voldemort.
One idea could be that in the alternate dimension, Voldemort won the war and Hogwarts is now a dark and dreary place where students are taught only dark magic. Another could be that the wizarding world is merged with the Muggle world in a strange way, like there are magical creatures running amok in London streets. Or perhaps in this alternate dimension, Harry was sorted into Slytherin and has to deal with a whole different set of challenges and prejudices.
One interesting idea could be that Harry decides not to destroy the Horcruxes but instead finds a way to redeem Voldemort. Maybe he discovers some long - lost magic that can purify the dark parts of Voldemort's soul. Another idea might be that Hermione becomes the true hero who finally defeats Voldemort with her vast knowledge of magic and out - of - the - box thinking. And there could also be a scenario where Ron sacrifices himself in a completely unexpected way to save Harry and the wizarding world, leading to a bittersweet ending.
In 'harry potter alternate school fanfiction', there are often ideas like a school that is floating in the air. This school could have its own set of rules and traditions. For example, students might have to use flying brooms to get from one class to another. Also, some fanfics might explore a school that is hidden deep underground. It could be a place where students study ancient and lost magics that have been forgotten by the wizarding world above. There could also be an alternate school where the students are sorted by their magical abilities rather than their personalities as in Hogwarts.
One idea could be an alternate universe where the ponies have different elemental powers. For example, instead of Twilight Sparkle being a unicorn with magic related to friendship, she could control time. This would change the entire dynamic of the stories and the relationships between the ponies.