Well, transformation is a big theme. Naruto might transform into a more demon - like form, and this transformation can bring both advantages and challenges. Another common theme is acceptance. Naruto has to accept the demon part of himself or his relationship with the demon. It could be a long and difficult process, but it's an important part of many 'naruto x demon fanfiction' stories. Also, there's the theme of the unknown. The demon element brings a lot of mystery, and Naruto has to explore this unknown side of his world.
A common theme is power. The combination of Naruto's existing powers and the added demon - related powers often leads to a new level of strength. Another theme is identity. Naruto has to figure out how being associated with a demon affects who he is. For example, in some fanfictions, he might struggle with being seen as a monster because of the demon connection.
Redemption is also a big theme. Naruto's demon side may have caused some problems in the past, and he tries to redeem himself in Sasuke's eyes. This theme adds a lot of emotional depth to the relationship between the two characters.
One common theme is power. Since Naruto is a demon and Kyuubi is a powerful being as well, the stories often revolve around how they use and control their powers. Another theme is partnership, showing how they work together.
Love and redemption are also prevalent. The fem demon may have a dark past or be seen as an outcast, but through her relationship with Naruto, she finds a path to redemption. And of course, the love that develops between them is a central theme that drives the story forward. For example, she might change her ways because of her love for Naruto.
One common theme is the exploration of their self - discovery. Since they are the same person in a sense, they can see different aspects of themselves. For example, one Naruto might be more confident while the other is more reserved, and they learn from each other.
Well, a frequent theme is the clash of cultures. The shinobi culture from Naruto and the more post - apocalyptic, tribal culture in The 100. Also, friendship and loyalty are common. Characters from both universes have to learn to trust and work with each other, and this forms the basis of many fanfictions.
The theme of redemption can also be found. Sometimes the demon in the fanfiction is on a path to redemption because of the influence of the reader. It gives the story a deeper meaning. Also, there's the theme of mystery. Demons are mysterious by nature, and the fanfiction often plays on that, with the reader trying to figure out the demon's true motives and nature as the story progresses. This keeps the readers hooked.
One common theme is forbidden love. Since demons and angels are often seen as polar opposites in the lore, their relationship is considered taboo.
Love against all odds is also very common. Society in these fanfictions usually frowns upon a relationship between a demon and a human, so the two characters have to fight for their love. And then there's the theme of power imbalance. The demon often has more power, and how they use or don't use it in the relationship is an important aspect.
Power is a common theme. The demon lord usually has great power, and the fem kyuu in the harem may be drawn to that power or gain power through their association with him.
Power is a common theme. Since the Raging Demon is a very powerful technique, many fanfictions focus on how characters gain, control, or are corrupted by this power. Another theme is heritage. The Raging Demon often has roots in the history of certain clans or ninja arts, so stories explore the connection between the present characters and their ancestors in relation to this technique.