In a more literal sense, if we think about a positive battery in a fanfic, it could be a new invention. Maybe a wizard in the Harry Potter world teams up with Tony Stark. They create a positive battery that uses both magic and technology. This battery could power a device that can nullify dark magic and the weapons of villains from both universes.
One example could be that the positive battery is used to power a portal. In the fanfic, this portal allows the Avengers to travel to the wizarding world of Harry Potter. It's a positive battery because it's a source of energy that enables a positive connection between the two universes, leading to new adventures and the ability to fight common threats together.
The petal could be a form of communication. In the fanfic, a character from the Avengers could find a petal with a message written on it in runes from the Harry Potter world. This could start an adventure where they have to find someone who can read the runes and decipher the message. Also, the petal could be used as a tracker. A wizard could use a spell on a petal to track the movements of an Avenger.
The poisoned apple could be a gift sent by a mysterious figure to one of the characters, like Iron Man. When he touches it, it starts to release a dark magic that affects his suit's technology. Harry Potter then detects the magic and tries to find a way to reverse the poison's effects.
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.
Well, he could be a new student at Hogwarts who's a bit older than the others. Since he has his unique way of seeing things, he might be sorted into Ravenclaw for his intelligence and ability to analyze. In the fanfic, he could start investigating some strange happenings around the school that seem related to the Forbidden Forest, using his profiling skills to figure out if there's a 'human' or rather a wizard or magical creature behind it all.
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.
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.
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.
First, you need to understand the characters well. Know their personalities, powers, and goals. For example, Thor is a powerful Asgardian with his hammer (or axe in Endgame), and Harry Potter is a young wizard with a strong sense of justice. Then, think of a plot that can bring these two universes together. It could be a crossover of villains, like Loki and Voldemort teaming up.