Another possibility is a story set in a world where the Mortal Kombat realm and the Harry Potter universe have merged. Cassie Cage, being a fighter from Mortal Kombat, could be teaching the Hogwarts students some self - defense moves. In return, the students could teach her about magic. This would lead to a lot of interesting interactions, especially when it comes to handling different kinds of threats in this new merged world.
A Cassie Cage and Harry Potter fanfic would be a blend of two very different worlds. Cassie, coming from a more modern and violent world of Mortal Kombat, would be a contrast to the more genteel and magical world of Harry Potter. In the story, she could be on a mission in the wizarding world, perhaps related to some Mortal Kombat - style tournament that has spilled over into the magical realm. She would have to navigate the complex rules of Hogwarts and the wizarding society while still being true to her own character. There could be some interesting interactions between her and the Hogwarts professors, who might be both intrigued and a bit alarmed by her. And of course, there would be lots of opportunities for action - packed scenes as she uses her skills to face the challenges in this new world.
First, do your research on both universes. Know the rules of magic in Harry Potter and the fighting styles and personalities in Mortal Kombat for Cassie Cage. Then, create an interesting plot that combines the two worlds in a believable way. For example, you could have a magical artifact that links the two worlds and Cassie has to retrieve it to save both universes. Make sure to develop the characters well. Cassie should retain her tough and determined personality, while also being able to adapt to the magical world. Harry and his friends should react realistically to Cassie's presence.
One idea could be a story where a Wendigo somehow enters the wizarding world of Harry Potter. Maybe it's brought in by a dark wizard who wants to use it as a weapon. The Trio - Harry, Ron, and Hermione - would then have to figure out how to defeat it using their magic and knowledge.
One idea could be that Hulk somehow gets transported into the wizarding world of Harry Potter. Maybe a magical experiment gone wrong by a Hogwarts student creates a portal that sucks Hulk in. He could then cause chaos as he's not used to the magic and strange creatures there.
There could be. Fanfic communities are full of creative minds. Some might see the potential in bringing together Cassie Cage's combat - related and modern - ish character traits with the magical and traditional world of Harry Potter. However, it may not be as common as some other crossovers. You may have to dig a bit deeper in fanfic archives. Try different search terms related to both characters and see what comes up.
Another concept is that the magic from the Harry Potter series starts to seep into the DC universe where the Justice League operates. This causes chaos as the laws of physics in the DC world are disrupted. The Justice League has to seek help from Hogwarts professors to understand and contain this new form of magic energy. Superman might be intrigued by the idea of magic being a new power source or a new threat he can't easily overcome with his super strength.
One idea could be that Tony Stark somehow discovers the wizarding world while working on a new technology. Maybe his sensors pick up some strange magical energy. He then meets Harry Potter and they team up to face a new threat that combines magic and technology, like a dark wizard using Stark Industries' tech for evil purposes.
One idea could be a fanfic where Harry Potter discovers a hidden portal in Hogwarts that leads him to the time of King Arthur. He could use his magic skills to assist Arthur in battles, perhaps using spells to defeat the enemies that Arthur's knights struggle with. For example, Harry could use the Stunning Spell to incapacitate a large group of invaders.
One idea could be to have nanomachines in the Harry Potter universe that are secretly created by a dark wizard. They could be used to control the minds of students, but the protagonists discover this and use magic inspired by LotR spells to combat it. For example, a Patronus charm with a LotR - like incantation to repel the nanomachine - influenced dark magic.
A third fanfic concept could involve a character from the Harry Potter series stumbling into Alma Wade's world. Let's say Hermione accidentally discovers a portal that leads to Alma's realm. She has to use her knowledge of magic and her wits to survive in this strange new place. Meanwhile, Alma Wade is intrigued by Hermione's magic and they start to learn from each other, with Hermione introducing wizarding spells and Alma sharing her unique powers.