A cool plot could be that Harry Potter, Ron and Hermione are transported to Adventure Time's world. They have to adapt to the strange and colorful environment. Harry's Patronus could be a very interesting sight in the Land of Ooo. They might team up with Finn and Jake to fight an evil wizard who has also found his way into this new world and is causing trouble with a combination of dark magic and the strange powers of Adventure Time.
Perhaps the magic from the Harry Potter universe seeps into the Land of Ooo. The Deathly Hallows start to cause chaos as different characters try to claim them. Marceline the Vampire Queen might be interested in the Elder Wand for its power. Meanwhile, Hermione could befriend Princess Bubblegum and they could work together to reverse the negative effects of the magic mix - up.
One idea could be that Finn and Jake from Adventure Time end up in the wizarding world of Harry Potter. They could use their unique abilities to help Harry and his friends in a dangerous quest, like finding a hidden Horcrux. Finn's sword could be useful against dark magic creatures, and Jake's shape - shifting might come in handy for sneaking around Hogwarts.
One idea could be that the cursebreaker discovers an ancient and forgotten curse in the Forbidden Forest that is slowly spreading and affecting the magical creatures there. They have to team up with Hagrid to find a way to break it before it reaches Hogwarts. Another could be a cursebreaker who is on a quest to find the origin of a powerful dark curse that was used in the past by Voldemort's followers and is now resurfacing in a different form. The cursebreaker travels to different wizarding locations around the world to gather clues.
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.
One idea could be that a time - twisting spell goes wrong and Harry ends up in the past during the founding of Hogwarts. He has to navigate the very different magical society then while trying not to change the future too much. Another could be that Voldemort discovers a time - twister spell and uses it to try and change the outcome of his battles with Harry. Or perhaps Hermione accidentally activates a time - twister device and she and Harry have to fix the timeline that gets all jumbled up.
One interesting plot could be the main character using time - traveling magic from the Harry Potter world to change key events in the Lord of the Time's story. For example, they might go back to prevent a certain dark wizard from obtaining a powerful time - related artifact.
One idea could be a story where the reader gets to choose whether Harry joins a secret magical society within Hogwarts that is dedicated to uncovering ancient spells. Another could be a plot where the reader decides if Harry follows a different path in his search for Horcruxes, like exploring a hidden underground lair in the Forbidden Forest.
One idea could be that Harry discovers a hidden room in Hogwarts that leads to different magical realms. He has to choose which realm to enter first. Another is that he gets the chance to change a major event in the wizarding world's history through his choices.
One idea could be a character going back in time to prevent a major dark wizard event that was overlooked in the original series. For example, they might try to stop the initial rise of Voldemort before he became so powerful.
Well, perhaps Harry travels back in time to the very beginning of the wizarding world. He discovers secrets about magic that have been long forgotten. He then uses this knowledge to make the wizarding world more inclusive and less divided by blood purity. Or he could time - travel during his own time period, like going back to his first year at Hogwarts over and over again to change small things that have big consequences in the future, like making sure Neville is more confident from the start.
Another plot could be that they travel back to the time when the Chamber of Secrets was first opened. Ginny, having been possessed by Tom Riddle in her own time, could use her knowledge to stop the young Voldemort from causing so much harm. Harry could also help by using his skills and knowledge of magic from his own time to combat the basilisk and keep students safe.
Well, a plot could be that a character from the future, like a descendant of Harry Potter, travels back in time to the time of the Triwizard Tournament. This character could have some knowledge of future threats and tries to prevent certain events during the tournament that could lead to greater disasters in the future. They might try to stop Voldemort's minions from using the tournament as a cover to carry out their evil plans. They could also try to make sure that Cedric Diggory doesn't die, which would have a huge impact on the wizarding community as he was a very popular and promising young wizard.