Another possible theme is transformation. Perhaps in these fanfics, Blue Willow and Angel undergo some sort of transformation, either physically or in their personalities. This could be related to the influence of the Black Cat or other elements within the story world of 'Black Cat by Tisienne'.
I'm not sure specifically which 'Black Cat by Tisienne' fanfics involving Blue Willow and Angel are out there. You could try searching on popular fanfic websites like Archive of Our Own. They have a wide range of fanfics and good search filters that might help you find what you're looking for.
One common theme is the exploration of their unfulfilled relationship. In the original series, they had a bit of an interesting dynamic that fanfiction often expands on romantically.
A common theme is the idea of protection. The angel often takes on the role of protecting the willow, whether it's from dark forces or from the encroachment of a human world that doesn't understand its magic. Another theme is the connection between the spiritual (the angel) and the natural (the willow). It shows how they can interact and influence each other in a fictional universe.
One common theme is power transformation. Since the symbiote gives the Black Cat new powers, many fanfics focus on how she adapts to these changes. For example, she might struggle to control the new strength or agility at first.
The contrast between their personalities is a common theme. Spiderman is more of the boy - next - door type with a strong sense of responsibility, while Black Cat is more of a free - spirit. Their interactions based on this contrast can lead to interesting storylines. Also, the theme of power and how they use it differently is often explored in fanfics.
A lot of non - adult Willow and Angel fan fictions also focus on mystery - solving. They are both involved in the supernatural world, so they might team up to solve a mystery that involves vampires, witches, or other creatures. This makes for an exciting and engaging plot. And sometimes, there's a theme of self - discovery. They both have complex identities and the stories can explore how they come to understand themselves better through their interactions with each other.
Another theme might be competition. They could be competing for resources like food or a particular resting place. This competition can drive the plot as they try to outwit each other. For example, the black bird might try to build its nest in a tree that the cat likes to nap under, leading to a battle of wits between them.
One common theme is friendship. Fanfics often explore how Willow and Lemon's friendship develops and endures through various situations. Another theme is the fight against evil. Since it's set in the Buffyverse, they usually team up to combat supernatural threats.
One common theme is redemption. Characters often use time travel to correct past mistakes or to save someone they couldn't before. For example, in some fanfics, a character travels back in time to prevent another character's death.
Another theme might be redemption. A character who has been through a 'black and blue' situation, like being beaten down physically or emotionally, might be on a journey of redemption. They could be trying to make up for past mistakes or heal from their traumas.