One idea could be a magical competition. They compete to see who can master a new and dangerous spell first. Buffy might use her traditional training and moral compass, while Faith could be more reckless in her approach, which would create a lot of tension.
For Buffy, her friends like Willow and Xander would be likely allies. Willow could use her magic to give Buffy an edge in any magical challenges, and Xander would be there for moral support and to offer his own brand of scrappy help. Giles, too, with his vast knowledge of the supernatural.
One great Buffy Faith fanfic is 'Faith's Redemption'. It delves deep into Faith's journey towards redemption. It shows how she overcomes her inner demons and tries to make amends for her past wrongs. The character development is really well - done.
Love and friendship are common themes as well. Whether it's platonic love between the two of them, or how their relationships with others are affected by their bond. In some fanfics, they learn to truly care for each other through difficult situations, which shows the power of friendship.
One idea could be a story where Buffy and Faith have to go undercover at a high - society event where the latex dress is part of their disguise. They end up having to work together to take down a vampire who is preying on the rich guests.
One common theme is redemption. Since Faith has a rather complex past, many fanfics focus on her journey towards redemption, often with the help of Mark and Buffy. Another theme is friendship. Despite their differences, Buffy and Faith can form an unexpected friendship, and Mark can be part of that bond. And of course, there's the theme of power. All three characters have different levels of power, and how they use or misuse it is often explored in the fanfics.
I really like 'The Lemon Grove Encounter' in the Buffy x Faith fanfic world. It's set in a unique location and shows how their relationship can be both tense and tender at the same time. The writing style is quite vivid, making it easy to picture the scenes in your head.
One of the main themes is the exploration of power. As Slayers, they both have power, and fanfics often deal with how they handle it, whether they compete or collaborate. Also, self - discovery is a big theme. They both are on a journey to figure out who they are, especially in relation to each other. For example, Faith might be trying to find out if she can be more like Buffy in some good ways, and Buffy might be learning from Faith's more reckless nature at times. It's a complex interplay of identities.
One common theme is redemption. Often, Faith is on a journey of redemption and Mark can play a role in that, along with Buffy's influence. Another theme is friendship. Despite their differences, the three characters form unique friendships. And power dynamics is also a theme. They all have different levels of power and how they interact with each other regarding power is explored in many fanfics.
Fanfiction.net is also a great place. It has a large collection of Buffy - related fanfics, including those focused on Faith. Just type in 'Buffy Faith' in the search bar and you'll get a list of relevant stories.