Another option is a crossover with the Harry Potter universe. The Shadowhunters could end up at Hogwarts, perhaps because there is some sort of dark magic that is spilling over into the mundane world that they need to stop. They would be amazed by the magic in the wizarding world, and there could be some funny moments as they try to understand how things work there, while also trying to keep their own identities and purposes a secret from the wizards.
A plot could involve a Twilight vampire falling in love with a shadowhunter. The vampire has to deal with the shadowhunter world's rules about the supernatural, and the shadowhunter has to come to terms with loving a vampire, which goes against their training. This could lead to a lot of internal and external conflicts as they try to be together.
One plot idea could be that the Shadowhunters need to find a way to defeat a powerful demon that can only be weakened by a special type of music. So they seek out the Glee club. The Glee club members are hesitant at first but then realize the importance of their role. Another idea is that a spell gone wrong in the Shadowhunters' world sends some of their key characters to the Glee club's world. There, they have to adapt to high school life while also trying to find a way back. And the Glee club helps them blend in and in return, they learn about the Shadow World.
One idea could be exploring a different origin story for a main character. For example, what if Jace had a different family background that changed his path in the Shadowhunter world. Another idea is a time - travel fanfic, where a character goes back in time and has to deal with the consequences of changing past events. It could also be interesting to write a fanfic about a character who has lost their powers and how they cope and try to regain them.
A fanfiction could explore the idea of a character from Twilight becoming a Shadowhunter. For example, Bella could discover she has Shadowhunter blood and has to leave the vampire - werewolf drama behind to learn a whole new set of skills and rules in the Shadowhunter world. It would be interesting to see how she adapts and how her relationships with Edward and Jacob change as a result.
Another plot could involve an OC who comes from a long - lost line of Shadowhunters. They bring back an ancient fighting style or a forgotten weapon. This OC could be on a quest to rediscover their family's past and in the process, they end up uncovering secrets about the Shadowhunter world that could change everything. Maybe they find a hidden lair with powerful artifacts that were thought to be lost forever.
One idea could be a crossover where the characters from both universes team up to fight a common supernatural threat that is a combination of demons from Supernatural and Downworlders from Shadowhunters. For example, a powerful demon has made a pact with a group of rogue vampires to take over both the human world and the Shadow World.
Another plot could involve a crossover with a post - apocalyptic world. The Boys find themselves in a world where society has collapsed, and they have to team up with the survivors against new threats. This could bring out new sides of the characters as they are forced to adapt to a completely different environment and face challenges they've never faced before.
One idea could be to cross over the characters from 'Percy Jackson and the Olympians' with those from 'The Kane Chronicles'. For example, the demigods and the magicians could team up to fight a common enemy that threatens both the Greek and Egyptian mythological worlds. Maybe a new dark force that has the power to disrupt the balance between the two realms.
One idea could be a crossover with Minecraft. The Terraria characters could find themselves in a Minecraft world filled with blocky landscapes and different mobs. They would have to adapt to the new building mechanics and find ways to combine their own abilities with the resources available in Minecraft.
One idea could be to cross it with a post - apocalyptic zombie world. The characters from Into the Badlands have to fight both zombies and their usual foes. They could use their martial arts skills in new ways, like using katana to slice through hordes of zombies.