One idea could be a crossover with the Harry Potter universe. The Shadowhunters could visit Hogwarts and interact with the wizards. They might have to team up to fight a common dark magic threat that combines the magic from both worlds.
Another interesting plot could be a crossover with the Twilight series. The vampires in Twilight and the vampires in the Mortal Instruments have different characteristics. A story could explore how the two groups react to each other, and how the Shadowhunters deal with the presence of both types of vampires in their world.
One idea could be that the Shadowhunters from The Mortal Instruments have to deal with the presence of vampires from Twilight in their world. Maybe they initially mistake the Twilight vampires for demons and chaos ensues. The Clary from The Mortal Instruments could form an unexpected alliance with Bella from Twilight to figure out how to co - exist.
One idea could be creating an OC who is a half - angel, half - demon. This would bring a unique perspective to the story as they struggle with their dual nature in the world of the Mortal Instruments. They could be on a quest to find a way to balance their two halves and in the process, interact with the main characters in unexpected ways.
Perhaps in a post - war scenario in both the 'Harry Potter' and 'Mortal Instruments' universes, Hermione and Draco decide to collaborate on a project to bridge the gap between the two magical communities. Along the way, they face opposition from traditionalists in both worlds who don't want the two communities to mix. Their relationship could develop from reluctant partners to something more as they fight for their cause. This would involve a lot of character growth, exploring new magical concepts, and dealing with the politics of both worlds.
One idea could be a crossover with Street Fighter. Picture Ryu facing off against Scorpion in a battle of fire and ice. Another could be a Mortal Kombat and DC Comics crossover, where Superman has to deal with the brutality of Mortal Kombat fighters. Or perhaps a crossover with the Castlevania universe, where the dark and gothic elements blend with Mortal Kombat's gore.
One idea could be Raiden being transported to an alternate universe where the rules of Mortal Kombat are completely different. He has to quickly adapt and find a way back home while facing new and strange opponents. Another could be Raiden teaming up with a Mortal Kombat character he's never interacted with before, like Johnny Cage, to defeat a common enemy that threatens both their worlds. Maybe this enemy has the power to disrupt the balance of the realms, which is Raiden's main concern.
One idea could be that the characters from RWBY are suddenly thrust into the Mortal Kombat tournament. Ruby could use her scythe skills in the battles, and her speed might give her an edge. Another could be that the villains from Mortal Kombat team up with the Grimm in RWBY. They could create chaos in both worlds. Also, Yang's strength could be pitted against some of Mortal Kombat's powerhouses like Raiden or Scorpion in some epic battles.
Well, one interesting aspect could be the unique character relationships that are often explored in fanfiction. Writers might take existing relationships from 'The Mortal Instruments' and add their own twists. For example, they could delve deeper into the friendship between certain characters and turn it into a more complex emotional bond. Another thing could be the new plotlines created. Maybe a character that was minor in the original gets a more prominent role and a whole new adventure is crafted around them. Also, the way fanfiction can re - imagine the magic and supernatural elements in different ways is quite fascinating.
A possible plot could involve Ermac's past being somehow linked to the crossover world. He discovers that an ancient evil in that world was involved in his creation. So, he goes on a quest to find out the truth and put an end to this evil once and for all. He might face many challenges along the way, like distrust from the locals of the crossover world who see him as a threat.
Jace Wayland is another great character. His charm and combat skills can add a lot to a crossover story. In a crossover with the Lord of the Rings, he could be seen as a kind of ranger, using his Shadowhunter skills to navigate Middle - earth and fight against the forces of Sauron.
Maybe a crossover with a magical anime. The nanbaka characters could suddenly find themselves in a world full of magic. Some of the nanbaka characters might be really good at using magic in unexpected ways, like Jyugo using his shackles in a magical battle.