A common theme is adventure. Stiles often goes on quests in these ancient settings, exploring unknown lands. Another theme is discovery, like finding new cultures or magical elements.
Power and its implications are also common. Stiles might gain some sort of power in the ancient world and has to learn how to use it responsibly. For example, in some fanfics, he gets the power to control ancient forces but has to deal with the consequences.
Supernatural elements are also very common. Since the show is about the supernatural, fanfics usually include things like werewolves, witches, and other magical creatures. Stiles' interaction with these elements is always interesting to read about.
There are often themes related to empowerment. Fem Stiles may be portrayed as breaking gender norms within the Teen Wolf universe. For example, she could be shown as being just as good or even better at solving mysteries or fighting supernatural threats. Friendship is also a big theme. How her relationships with the other characters, like Lydia or Allison, change or stay the same in a different gender context can be an interesting exploration. Sometimes, there are also themes related to family, especially how her relationship with her father might be different as a female.
One common theme is protection. Since Stiles is in a baby form, other characters often have to protect him from the various dangers in the supernatural world. Another theme is cuteness. Writers usually focus on the adorable things baby Stiles does like his little giggles or how he toddles around.
One common theme is the relationship between the cheerleader and Stiles. It often involves a budding romance or a strong friendship.
One common theme is power. Since Stiles is an alpha, the story often explores how he wields his power, whether it's for good or to face his enemies. Another theme is loyalty. His friends' loyalty to him and vice versa is usually a big part of the story.
One common theme could be the challenges and joys of impending parenthood. Stiles might face difficulties in preparing for the baby while dealing with his own life situations.
The theme of redemption can also be seen. Theo, especially, might have a past that he needs to make up for, and his relationship with Stiles can be part of that journey. And of course, the theme of love against all odds. They face various obstacles, but their love perseveres.
One common theme is loyalty. Their friendship is often depicted as unwavering, no matter what difficulties they face. For example, in many fanfics, they will always have each other's backs during battles or when dealing with the mysteries in their town.
Another theme is the fear and excitement of the impending parenthood for both Sterek. Stiles may have his own insecurities about being a good parent, and Derek has to support him through that. It's often about their journey together in facing this new chapter in life. For example, in some fanfics, they deal with how to prepare for the baby, like getting a nursery ready. Also, there's often the theme of acceptance from their friends and family, which can be a source of both conflict and support in the stories.
One common theme is the power of friendship. They often start from a place of mistrust or misunderstanding and gradually become friends or more. Another theme is the idea of redemption, especially for Peter.