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.
Well, Kassandra is a really cool and complex character. Her story in the game is so rich that it leaves a lot of room for fanfiction. People are drawn to explore her further, like her relationships with other characters or what she does after the main game events.
Power and its consequences is a frequent theme. Given Kassandra's abilities in the game, fanfictions might explore how she uses or misuses her power. Also, friendship and loyalty are common. She forms bonds with various characters in the game, and in fanfiction, these relationships are often deepened or tested. For example, her relationship with Barnabas is often a central focus in many stories.
Another interesting one is 'Kassandra, Champion of the Creed'. In this fanfiction, Kassandra becomes a central figure in the Assassin's Creed organization much earlier than in the main storyline. It's full of action and shows her making important decisions for the Creed.
There are quite a few out there. For example, 'Kassandra's Redemption' is a fanfiction that focuses on Kassandra's internal struggle with her past actions. It gives a deeper look into her character. Also, 'Kassandra and the Mysterious Artifact' is exciting as it has her on a quest to discover the power of a long - lost artifact. This one has a lot of action and mystery elements.
A good plot twist could be that Hades was actually being controlled by an ancient evil force all along. Kassandra discovers this when she notices his strange behavior. She has to find a way to break the control and save Hades as well as the Underworld from this hidden threat.
One interesting concept could be a crossover with a fantasy world like Middle - earth. Imagine an assassin having to navigate the perils of Mordor while still maintaining their creed. Another concept might be a crossover with a historical event not directly related to the main Assassin's Creed timeline, such as the sinking of the Titanic, where assassins could be on board for a secret mission.
Writing a good Assassins Creed crossover fanfic is all about balance. You have to balance the elements of Assassin's Creed, like the hidden blade and the assassin's parkour skills, with the elements of the crossover. Let's say it's a crossover with a detective story. You can't overshadow the detective's investigation skills with the assassin's combat abilities. Also, create believable relationships between the characters. If an assassin teams up with a detective, there should be some initial mistrust that gradually turns into respect and cooperation as they work towards a common goal.