One idea could be a bounty hunter who is tasked with tracking down a dark wizard who has escaped from Azkaban and is hiding in the Muggle world. The bounty hunter has to use both magic and knowledge of Muggle ways to find the wizard.
Well, think about a story where the Force in Star Wars is combined with magic from Harry Potter. A bounty hunter could be caught in the middle of this power struggle. Maybe a wizard - turned - bounty - hunter has to use both magic and his blaster to complete his missions. Or perhaps a story where the Death Eaters are somehow involved in the bounty hunting business in the Star Wars galaxy, bringing their dark magic to a new and unfamiliar place.
Another could involve a bounty hunter who has a personal vendetta against a group of Death Eaters. This hunter might team up with Harry Potter and his friends, bringing in their own set of skills and knowledge about the underworld of the wizarding world. They could be really good at disguises and infiltrating Death Eater hideouts. For example, they might use a special potion to change their appearance and blend in with the bad guys to gather information.
Well, perhaps Harry could be hunting down dark wizards who have escaped from Azkaban and are causing chaos in the wizarding world. He might use his skills from his battles against Voldemort, like his knowledge of spells and his ability to think on his feet.
Perhaps a group of wizards and witches form an alliance with Monster Hunters to stop a powerful dark magic - infused monster that is terrorizing both the wizarding world and the Monster Hunter's territory. They have to exchange knowledge, like the wizards teaching the Monster Hunters about potions for healing and the Monster Hunters showing the wizards how to use their unique weapons more effectively.
A plot could involve a group of witch hunters infiltrating Hogwarts. They think there are dark witches there, but they end up getting lost in the magical castle and start to see the beauty and positive aspects of magic. As they interact with students and teachers, they have to re - evaluate their views on witches and magic in general.
Another interesting plot point is the power struggle. In some fanfics, the bounty hunter might be very powerful in their own universe, but in the Harry Potter world, there are powerful wizards and dark forces. How they navigate this new power hierarchy, whether they try to dominate or find a place within it, makes for a great story. Also, the idea of a bounty hunter having to deal with the Ministry of Magic's laws and regulations which are very different from the lawless areas they might be used to in Star Wars is really engaging.
One idea could be a bounty hunter who is initially after a character from RWBY but then discovers they are on the wrong side. For example, hunting Ruby at first because she's thought to be a dangerous criminal, but as the story unfolds, realizes she's actually a hero. Another idea is a bounty hunter who has a special weapon or power unique to the RWBY universe, like a weapon that can transform into different types of dust - powered tools. A third idea could be a bounty hunter who teams up with the RWBY gang to take down a greater threat, like a powerful and elusive criminal organization.
A fanfiction could focus on a character from Harry Potter who is a secret Hunter x Hunter fan. They could be constantly comparing the two worlds in their mind. Like how the Sorting Hat in Harry Potter is similar to the way Hunters are selected in Hunter x Hunter. Or how the Forbidden Forest in Harry Potter has some similarities to the Dark Continent in Hunter x Hunter.
Well, you could have a fanfic where Voldemort uses DND - style dark magic to open portals to other dimensions. The Order of the Phoenix has to quickly learn DND - style combat and magic rules to fight off the new threats that come through these portals. Harry could be sent on a special mission to find a DND - style mage who can teach them the counter - spells. Meanwhile, the Death Eaters are also trying to master this new form of magic for their own evil purposes. This would create a really exciting and unique story.
Perhaps a plot where the protagonist can choose to either side with the Ministry of Magic during a time of political unrest within the wizarding world or join a rebel group. This would involve different magical battles and moral dilemmas depending on the choice. In the 'harry potter cyoa fanfic', this could show different aspects of the wizarding society that we don't often see in the original books.