One idea could be exploring what would happen if a new, mysterious student joined Hogwarts during the Triwizard Tournament. This student could have a hidden agenda related to the Dark Arts and start influencing the events of the tournament in unexpected ways.
Some fanfictions might focus on different relationships. Maybe a story where Harry forms a closer bond with Cedric Diggory before the tournament, changing the dynamics of the events. It could show how they might have worked together more or influenced each other's strategies.
One interesting plot could be an alternate version of the Triwizard Tournament where the tasks are even more dangerous and magical creatures from other regions are involved. For example, instead of just a Hungarian Horntail, there could be a whole nest of them to face in the first task.
In many of these fanfictions, Norbert's relationship with Hagrid is often explored further. For example, in some, Norbert misses Hagrid deeply after being sent away. Another interesting plot point can be Norbert's impact on the magical environment around Hogwarts. Since he's a dragon, his presence can cause all sorts of magical disruptions.
One idea could be a crossover where characters from both universes meet. For example, Harry Potter could encounter Eragon and they could learn from each other's magic systems. Maybe Harry could teach Eragon about some of the more complex spells in the wizarding world, while Eragon shows Harry how to bond with a dragon in a more in - depth way.
You could also create a plot where a character who thought they had no connection to the Potter heritage suddenly discovers they do. This revelation could change their entire life. They might have to deal with newfound magical abilities that come with the Potter bloodline, and also face the enemies that the Potter family has made over the years, like some die - hard Death Eaters who still hold grudges.
A great plot idea might be a Craftsmith who is actually a descendant of an ancient wizarding family with long - lost spells and abilities. This Craftsmith uses their skills to recreate those spells and in the process, uncovers a plot by dark wizards to use these forgotten magics for evil. As the story unfolds, the main characters have to stop the dark wizards while also learning about the new and old magics.
One idea could be that the orphanage is a hidden place for magical orphans with special powers. The orphans form a secret society within the orphanage to protect themselves from dark wizards who want to use their powers. They learn magic on their own, using old books they find in the attic of the orphanage.
One plot idea could be the characters from Harry Potter learning about the different types of power and ranking systems in DxD. For example, Harry might be intrigued by the concept of Sacred Gears and try to figure out if there's anything similar in the wizarding world.
One idea could be an Acromantula befriending a Hogwarts student who is an outcast. The student discovers the Acromantula has a unique ability that could help in the fight against Voldemort. So they team up secretly.
One idea could be Harry Potter arriving in Skyrim and using his magic to combat the dragons. Maybe he teams up with the local mages and they learn from each other's magical techniques.