One plot could be Kassandra as a mentor. She could be teaching young warriors the ways of combat and survival. It would show her wisdom and experience in a positive and family - friendly way.
Perhaps she gets hurt while trying to protect someone she cares about. She could be shielding a friend from an attacker's blow, and in the process, take the brunt of the injury herself. This would then set up a story of her recovery and the revenge she might seek on the one who caused the harm.
It could also be due to differences in their beliefs. Brasidas is a Spartan, with his own set of values and loyalty to his city - state. Kassandra, on the other hand, has a more individualistic view having traveled far and wide. Their different outlooks could lead to a clash. For example, if there was a decision to be made about how to deal with a new threat in the region, Brasidas might want to handle it in a traditional Spartan way, while Kassandra could have a more unorthodox approach.
Some fanfictions focus on the idea of time travel or alternate universes. Maybe Kassandra is transported to Eivor's time, or vice versa. This allows for unique scenarios where they have to adapt to a completely different world while also dealing with their growing relationship.
It's really hard to say. Brasidas is a formidable Spartan warrior with great strength and military training. However, Kassandra has the advantage of being a misthios, with a wide range of skills and experiences from all over the world. If the fight was in a more traditional, open battlefield, Brasidas might have the upper hand due to his experience in phalanx warfare. But if it was in a more urban or diverse environment, Kassandra's adaptability could give her the win.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - made stories, including Kassandra x Eivor ones. Just use the search function on the site to look for it.
One great 'AC Odyssey' Kassandra fanfiction in the Alpha timeline might be those that explore her early days as a misthios. They could focus on her first encounters with important characters and how she developed her skills.
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.
The character of Kassandra herself can make a novel interesting. If she is depicted as a strong - willed seer, her struggle with her prophetic visions and how she is treated by those around her can be captivating.