Perhaps 'Over the Rainbow' is a secret realm that holds the key to a powerful magic. In the fanfic, a group of characters including some from 'The Hobbit' and some from 'Harry Potter' set out on a quest to find this realm. Once there, they discover that the magic there can help them in their battles against evil, like Sauron in Middle - earth and Voldemort in the wizarding world. The colors of the rainbow could also represent different types of magic that they have to learn to harness.
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 idea could be a crossover where a character from 'The Hobbit' accidentally travels through a magical portal 'Over the Rainbow' and ends up in the world of 'Harry Potter'. They could then have to navigate the wizarding world, using their dwarven or elven skills to deal with the new challenges.
In some fanfiction, dragons in the Harry Potter and Hobbit universes might be combined in terms of their magical abilities. For example, a dragon in a crossover fanfic could have the intelligence of a Hobbit - dragon like Smaug but also be able to use magic like the dragons in Harry Potter. They could be used as powerful plot devices, perhaps guarding a powerful magical artifact that both wizards from Harry Potter and characters from The Hobbit are after.
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.
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.
In this fanfic, it could be a really emotional and fantastical twist. Maybe it's a magical realm beyond our imagination where the funeral takes place.
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.