One interesting possibility is a universe where Voldemort won. It would be a dark and desolate world. The remaining Order of the Phoenix members would be in hiding, and Hogwarts would likely be a place of indoctrination rather than learning. Another could be a universe where Harry was sorted into Slytherin. He might have had a very different set of friends and challenges, and perhaps a more complex relationship with Draco Malfoy.
Well, in some fanfics about traveling alternative universes in Harry Potter, there are universes where magic is used in completely different ways. For example, instead of using wands, witches and wizards might use their minds directly. Also, there could be a universe where the four houses of Hogwarts are not as divided as in the original story. Students from all houses work together more closely, and there are no rivalries like the Gryffindor - Slytherin feud.
In many fanfics, it often involves some sort of powerful magical artifact. For example, a special enchanted mirror that can show different universes and, with the right spell, can be used to step into them. Another way could be through a time - turned - portal. Since time and universes are often intertwined in magical theories, a malfunctioning time - turner might accidentally open a gateway to an alternative universe.
There could be a fanfic where Snape survives the Battle of Hogwarts. After the battle, he and Harry have to come to terms with their past relationship. Harry realizes how much Snape sacrificed for him and the wizarding world. They start to build a new relationship, with Snape helping Harry to understand some of the more complex and dangerous aspects of magic that were once only known to Snape and Voldemort.
A plot that I find very intriguing is when a character travels back in time to prevent the death of a loved one. For example, in some fanfics, Harry travels back to save Sirius. This creates a lot of tension as he has to deal with the time - turner rules and the possible consequences of changing the past.
Some fanfics might explore Nagini's connection to the other Horcruxes. Since she was the last Horcrux to be destroyed, perhaps there was some kind of communication or power exchange between her and the other Horcruxes that we didn't see in the books. This could add a whole new layer of mystery to the story.
One great aspect of Salazar Slytherin x Harry Potter fanfiction is the exploration of different time periods. Writers can create scenarios where Harry somehow travels back in time and meets Salazar Slytherin. This can lead to all kinds of interesting interactions. Maybe Harry tries to change Slytherin's views on muggle - borns, or perhaps Slytherin sees something in Harry that makes him question his own beliefs. It's a really fascinating concept that allows for a lot of creativity in the Harry Potter fanfiction world.
Well, in some older Harry Potter time travel fanfiction, the plot might involve Harry traveling back to his own past to fix his mistakes. He might have regrets about how he treated certain people or how he made certain decisions. So, he goes back to make amends. This can lead to some really heartwarming and also tense moments as he tries to change things without causing more harm. For instance, he might go back to when he first met Hermione and be nicer to her from the start. This can then change their relationship in unexpected ways throughout the story.
Harry Potter himself is still very popular in these fanfics. After all, the whole concept often revolves around him in some way, whether it's his journey through different universes or his interactions with other versions of himself or different characters from other universes.
A really interesting plot twist would be if Harry Potter was somehow transported to Gotham as a child and raised there. He would grow up with a different set of values and skills, and when he discovers his magical abilities later on, he has to balance his newfound magic with the street - smart tactics he learned in Gotham. This would completely change his character and how he approaches the fight against Voldemort. And when Batman eventually meets him, he's not the typical Harry Potter we know from the books. It could lead to a whole new dynamic between them.
One interesting plot twist could be that Harry goes back in time and accidentally changes the sorting of the houses. This would lead to different friendships and rivalries. For example, if he made it so that Draco was sorted into Gryffindor, it would completely change their dynamic. Another could be that he changes the outcome of a Quidditch match in the past, which then has a ripple effect on the school's morale and subsequent events. Maybe Gryffindor loses a crucial match they were supposed to win, and it makes them more determined in other aspects.
In some fanfictions, like 'The Unforeseen Consequences of Time Travel', a plot twist could be that when Harry and Hermione go back in time to save someone, they end up causing more harm than good. This completely flips the expected outcome on its head and makes for a very engaging read.