Well, a cool crossover might involve the Assassin's Creed settings with Pokemon. Imagine exploring the ancient cities from Assassin's Creed games, but instead of just assassins, there are trainers with their Pokemon. Some Pokemon could blend into the environment like the assassins do, and they could help in combat or gathering information in a very unique way. For instance, a Flying - type Pokemon could scout areas just like the assassins' eagle vision in a sense.
In some Adventure Time Assassin's Creed fanfictions, there are crossovers where the Assassins' Creed artifacts are brought into the Land of Ooo. For example, the Apple of Eden might end up in the hands of Princess Bubblegum. She could use its power to try and bring more order to her kingdom, but it might have unforeseen consequences as the power of the Apple is often a double - edged sword in Assassin's Creed lore. Also, Marceline might be interested in the power of these artifacts as she has her own complex relationship with power and immortality.
In some fanfictions, the Isu from Assassin's Creed could be related to the gods in Percy Jackson. Maybe the Isu were the predecessors of the Greek gods. This would lead to a whole new set of adventures as characters from both universes try to figure out their shared history. Also, the hidden blades in Assassin's Creed could be enchanted with Greek magic in the fanfiction, making them even more powerful and useful in battles against both mortal and mythical foes.
One interesting crossover could be a character from Assassin's Creed using their skills in the Star Wars universe. For example, an Assassin might use their stealth abilities to infiltrate a Sith lair. They could use their hidden blade in combat against Sith warriors or droids. Another aspect could be the use of the Force in an Assassin's Creed - like way. Maybe a character discovers a new form of the Force that allows them to move like an Assassin, quickly and quietly.
One interesting crossover could be the characters from RWBY using the Assassin's Creed parkour skills. Ruby, with her speed, could be an amazing assassin, running across rooftops in Vale like an Assassin. Another could be the introduction of the Pieces of Eden into the RWBY world, causing chaos as different factions fight over their power.
One of the most interesting crossovers could be the combination of RWBY's unique combat styles with Assassin's Creed 4's stealth and parkour elements. For example, Ruby's scythe - wielding skills could be adapted to the pirate - themed world of Assassin's Creed 4. She could use her speed and acrobatics to traverse the ship masts and islands in a similar way to how Assassins move in the game.
One interesting crossover could be the characters from Fairy Tail using Pokemon in their battles. For example, Natsu could have a Fire - type Pokemon like Charizard to enhance his already powerful fire - based attacks. Another could be Lucy having a Psychic - type Pokemon to help her with her Celestial Spirit magic, perhaps a Gardevoir that could work in tandem with her spirits.
In this type of fanfiction, there might be a crossover where a Pokemon character somehow enters the Naruto universe. For instance, Pikachu could end up in the Hidden Leaf Village. When the Nine - Tails senses Pikachu's electric energy, it could be intrigued. This could lead to an adventure where Naruto and Pikachu have to work together. Maybe Pikachu helps Naruto control the Nine - Tails' power better using its own unique skills, like how it can store and release electricity.
One interesting crossover could be the combination of Naruto's ninja skills with Fairy Tail's magic system. For example, Naruto's shadow clone jutsu combined with Fairy Tail's fire magic could create a really powerful and unique attack. Another could be the characters from Naruto interacting with the guilds in Fairy Tail. Maybe Naruto and his friends could join a Fairy Tail quest and learn about their magic along the way.
One interesting idea could be exploring the backstory of a minor character in the game. For example, you could write about the early days of a random assassin in training, their struggles, and how they overcame them. Another idea is to create an alternate universe where the Templars and Assassins have a different kind of relationship, like a truce that changes the whole dynamic of the story. Or you could focus on a specific historical event that was only briefly touched upon in the games and expand it from an Assassin's Creed perspective.
One interesting Assassin's Creed fanfiction could be about an assassin in a modern - day city trying to uncover a secret Templar plot that involves big corporations. They use their skills in stealth and combat to gather evidence while also dealing with normal life problems.