How about a crossover with the Mass Effect universe? The protagonist of Assassin's Creed Odyssey could find themselves on a Citadel station. They would be amazed by the advanced technology there. They could team up with Commander Shepard to fight against the Reapers. The assassin's combat skills and knowledge of ancient technology could provide a unique perspective in the fight against the cybernetic threat of the Reapers.
One idea could be a crossover with the Witcher series. Kassandra or Alexios could meet Geralt in a world where the Isu technology has somehow intersected with the magic of the Witcher world. Maybe they team up to fight a common enemy that is using both magical and ancient technology to cause chaos. Another plot could involve a crossover with the Lord of the Rings universe. The hero from Assassin's Creed Odyssey could be transported to Middle - earth and get involved in the War of the Ring, using their combat skills and stealth abilities to help the Fellowship in unexpected ways.
A more character - driven plot could be about the relationship between Kassandra or Alexios and their family members. You could write a fanfic that delves deeper into their feelings towards their mother, father, or siblings. What if there was a secret that their family was hiding from them all along? This could add a whole new layer of drama and mystery to the story.
A great plot for an Assassin's Creed Odyssey fanfic could be centered around a love story. The main character could fall in love with an enemy, like a high - ranking Athenian while being a Spartan assassin. They would have to navigate their feelings while also being true to their respective causes. It would add a lot of drama and tension. Also, a plot about a hidden cult within the game's world that the main character discovers and has to take down. This cult could be involved in dark magic or ancient rituals that are threatening the stability of the Greek world in the game.
Well, a cool plot idea might be the character time - traveling and accidentally bringing a modern - day weapon with them. They then have to figure out how to use it without disrupting the historical timeline too much. Or perhaps they travel to a time when the Isu, the precursor race in Assassin's Creed, were more active and interact with them. This could lead to uncovering lost technology or knowledge.
For an Assassin's Creed crossover fanfic, consider a crossover with the Legend of Zelda series. The Assassins could explore the vast lands of Hyrule, facing off against Ganon's minions. They could learn some of the unique abilities and use the weapons in the Zelda world. Also, a crossover with the Pokemon world would be interesting. The Assassins might be on a quest to stop an evil organization that is misusing Pokemon for their own dark purposes, and they would have to catch and train Pokemon to help them in their journey.
One idea could be a crossover with the Lord of the Rings universe. The Assassins could be sneaking around Middle - earth, using their skills to navigate the dangerous terrains and perhaps even facing off against the forces of Sauron. Another idea is a crossover with the Harry Potter world. Imagine an Assassin infiltrating Hogwarts, using their hidden blade to protect the wizarding world from dark forces that the wizards might not be able to handle with just magic.
Another angle for a fanfic about Xenia could be her relationship with the protagonist. Perhaps they form an unlikely alliance based on mutual respect and a common goal. They could learn from each other's skills and experiences, and this could lead to a strong bond between them as they journey through the game's world together.
Perhaps the Sphinx has a connection to an alternate dimension in the Assassin's Creed Odyssey universe. The protagonist, while trying to solve its riddles, accidentally gets transported there and has to find their way back while also dealing with the strange creatures and challenges of that dimension.
One idea could be a crossover with Pirates of the Caribbean. The Assassin could team up with Jack Sparrow to find a hidden treasure that is also related to an ancient Piece of Eden. They'd face challenges like sea monsters and rival pirates.
One idea could be a story where Kassandra, during one of her adventures, stumbles upon a mysterious portal that leads her to the Underworld. There, she meets Hades. Hades is initially suspicious of her but soon realizes she has no ill intent. They could form an unlikely alliance to deal with a threat that endangers both the mortal world and the Underworld.
One possible plot could be that the protagonist from Assassin's Creed Odyssey somehow gets transported to the world of PJO. Maybe they end up in Camp Half - Blood and are mistaken for a demigod at first. They then have to use their assassin skills to survive the various challenges in this new world, like facing off against mythical creatures that are different from the ones in their own world.