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.
It's a bit complex to define Kassandra as simply anti or pro religion. The novel presents her with various situations that shape her nuanced views on the matter. Sometimes she questions religious practices, but at other times, she may find solace or guidance in them.
The First Blade story and Kassandra's connection is quite complex. It involves various plot elements and character interactions that shape the overall narrative.
A common theme is power. Hades is a powerful god, and Kassandra is a strong warrior. Their interactions often involve power dynamics, like in some fanfictions where Kassandra has to outwit Hades' power. Another theme is fate. Given Hades' role in the Greek mythology regarding the afterlife, fanfictions often explore how Kassandra's fate is intertwined with Hades' actions or decisions.
You can start by looking on fanfiction platforms like Archive of Our Own. Search for Kassandra and female reader, and then filter out the smut tags. There are likely to be many great stories focusing on their friendship, adventures together, or even just their interactions within the game's lore.
One great piece of 'AC Odyssey Hades and Kassandra fanfiction' is 'The Encounter in the Underworld'. It delves into Kassandra's journey into the underworld where she meets Hades. The author does a wonderful job of portraying Kassandra's courage in the face of the god of the underworld. Another one is 'Hades' Offer' which shows an interesting interaction between them where Hades presents Kassandra with a deal that could change her fate.