You can make taxes a part of the conflict in your fanfic. Let's say a new tax law is passed that taxes all potions ingredients. This would cause problems for potion - makers like Severus Snape. He might have to find cheaper ingredients or raise the price of his potions, which could lead to disputes with students or other wizards who rely on his potions. You could also write about how the tax affects the black market for ingredients that some wizards might turn to in order to avoid the tax.
One way is to create a new Ministry department that deals with magical taxes. You could write about how this department audits wizarding businesses. For example, Ollivander's wand shop might be audited for taxes on the sales of wands. Another way is to introduce a tax on magical creatures. Hagrid might have to deal with the new tax regulations for his care of magical creatures at Hogwarts.
Taxes in Harry Potter fanfic might not be a common theme, but they could add an interesting layer of realism. If there are taxes on magic use, it could lead to some characters having to find ways around it or deal with the consequences of not paying. Maybe certain spells are taxed depending on their power or rarity, which could create conflicts between wizards who use more powerful magic and those who can't afford to pay the tax.
You could make the succubus drain be part of a curse that was placed on Hogwarts long ago. Maybe it was put there by a dark wizard who wanted to weaken the school's magic. In the fanfic, the characters start noticing that students are getting weaker and tired all the time. They then discover the source is a succubus draining their energy, and they have to search through old books and magic spells to find a way to break the curse.
Well, 'Harry Potter' candy can be used as a plot device in fanfics. For instance, a poisoned candy could be sent to a character, leading to a mystery of who the culprit is. It could also be a way to show friendship, like when a character gives their favorite Honeydukes candy to another as a sign of loyalty.
The defenestration spell could be incorporated as a key plot device. For instance, if there's a mystery about a series of 'window - related' incidents in Hogwarts, and Harry suspects the defenestration spell is involved. He then starts to investigate who might be using it and for what purpose.
It could be a reference to a certain character's look or attitude. Maybe there's a new character in the fanfic who has a steely gaze, and the author describes it as 'blue steel' to give a sense of mystery and cold determination. This character could be an ally or an enemy to Harry and his friends, adding a new layer of intrigue to the story.
They can be incorporated as new magical creatures in the Forbidden Forest. Maybe they have their own hidden enclaves there and interact with Hogwarts students who stumble upon them. For instance, a faery hero could offer help to a lost student by using its faery magic to guide them back to the castle.
It could be a magical place that characters from both universes are drawn to. For example, Frodo from 'The Hobbit' might be lured to 'Over the Rainbow' by a strange magic, and there he meets Harry Potter. They then have to work together to return to their respective worlds.
One way is to have it as a guide. For instance, a young wizard or witch could be lost in a forest near Hogwarts, and a blackbird flying overhead could lead them to safety, like to a hidden passage or an old, forgotten hut that holds some important magical item.
Magic wands can be used. The person asking can wave their wand and make a magical display of lights and sparkles that form the words of the promposal. For example, a simple 'Wingardium Leviosa' spell can be used to levitate the promposal sign in front of the person being asked.
The Gorgon could be incorporated as a new threat that emerges from an ancient part of the wizarding world. For example, if a dark wizard discovers a way to control the Gorgon, it could be sent to Hogwarts to cause chaos. Harry would then have to figure out a way to stop it without getting turned to stone.