The magic in Harry Potter is based on spells, potions, and magical creatures. In a fanfiction, Geass could be seen as a special type of spell or curse. For example, a Geass could be cast like a spell, but with different rules. Maybe it requires eye contact like in Code Geass, and the effects are more long - term and powerful than normal spells. The magic system in Harry Potter could also be used to counter or enhance Geass. For instance, a powerful anti - curse spell could be used to break a Geass if it's seen as a form of dark magic.
In a fanfiction, the combination could work like this. Lelouch's Geass, which is a power of the mind, could interact with the magic related to the mind in the Harry Potter world, such as Legilimency and Occlumency. Maybe Lelouch could use his Geass to break through the Occlumency defenses of a powerful wizard. On the other hand, the magic of Harry Potter, like the Patronus charm, could be used to protect against the negative effects of Geass. If someone was being influenced by Geass against their will, a Patronus could drive the Geass influence away.
You could map the different spells in Harry Potter to D&D - style abilities. For example, the Patronus charm could be like a powerful defensive ability that requires a certain amount of mana (or magical energy).
Magic spells from Harry Potter can be given Magic the Gathering - like mechanics. For example, the 'Expecto Patronum' spell could cost a certain amount of mana (like white mana for its purity aspect) and be used to protect against dark forces, which in the fanfiction could be related to certain types of Magic the Gathering creatures like demons.
In some fanfictions, magic can be used to ease the difficulties of pregnancy. For example, spells might be cast to ensure the health of the mother and the unborn child in the harem situation. Magic could also be the reason for the harem formation, like a love - binding spell that brings all the characters together and leads to pregnancy.
Well, in Narnia, there's the magic of Aslan which is more of a divine and nature - based magic. In Harry Potter, it's more about spells and potions. You could have a situation where a wizard tries to learn the magic of Aslan. For example, Hermione might be intrigued by the magic that brings Narnia to life and tries to study it. She could combine her knowledge of wizardry with the Narnian magic to create new spells.
The magic in Harry Potter is mainly spell - based, while in The Dragon Prince it is more about primal sources. In fanfiction, a character could try to combine the two. For instance, a wizard could use a spell to enhance the power of a primal source connection. They might use the Wingardium Leviosa spell to levitate an object charged with primal magic.
You could create a new type of magic that combines the two. Let's say there is a character in the fanfiction who is half - wizard from Harry Potter and half - something related to Black Butler (maybe has a demon ancestor). This character has a unique magic that is a blend of spell - casting and the ability to make deals with supernatural forces. Their spells might have a different effect because of the demonic influence. For instance, a simple levitation spell could also cause a small amount of fear in those around them because of the dark undertones from the Black Butler magic.
The spells in Harry Potter could be combined with the Thu'um of the Dragonborn. For instance, a wizard could cast a spell to enhance the power of the Dragonborn's shout. Or the Dragonborn could use his shouts to disrupt spell - casting of dark wizards.
In a crossover, the magic of apparition could be a source of horror. A character could apparate to a wrong place that is filled with Creepypasta - inspired horrors. Maybe a place that looks like Hogwarts but is full of twisted versions of the people and things they know. The magic of divination could also be combined. Instead of normal prophecies, a seer could start seeing visions of Creepypasta - like events that are about to happen in Hogwarts. And the magic of the Room of Requirement could be used in a really dark way. Instead of providing what the user needs, it could trap a character in a never - ending nightmare - like scenario filled with Creepypasta elements.
The magic systems could be combined in terms of magical education. Imagine a school where students from both universes learn. They would study the basics of both magic systems. So, a student would learn about the seven basic magecraft in Fate Stay Night as well as the different types of spells in Harry Potter. This would lead to a new generation of characters who could blend the two magics in creative ways. For example, using the teleportation magic from Fate Stay Night in combination with a Disillusionment Charm from Harry Potter for a stealthy escape.