The OC could be a former assassin who has given up his old ways. He crosses paths with Yona's group when they are being hunted by a powerful enemy. He uses his skills to protect them. Yona is hesitant at first to trust him, but as she sees his genuine remorse for his past actions and his dedication to their cause, she accepts him into the group. Together, they face the challenges of not only the external threats but also the internal struggles within the group as they adjust to having an ex - assassin among them.
Another idea is that the OC is a young artist who has the ability to see the true nature of people through their auras. When he meets Yona, he sees a brilliant and pure aura around her. He decides to follow Yona and her group, documenting their journey through his art. His artworks start to have a magical effect, inspiring the people of Kouka to support Yona's cause. Yona, in turn, is inspired by the OC's creativity and positive outlook, and they form a unique bond.
One idea could be an OC who is a healer from a far - off land. She meets Yona and the group and uses her unique healing powers, which are based on ancient tribal knowledge, to help them when they're injured. For example, she might be able to heal not just physical wounds but also emotional traumas to some extent.
For an 'akatsuki no yona x oc' fanfic, let the OC be a skilled archer named Lyra. Lyra had heard tales of Yona and her quest. One fateful day, their paths crossed during a battle against a group of bandits. Lyra's arrows flew true, helping Yona's group fend off the attackers. Yona was impressed by Lyra's marksmanship and invited her to join their journey. Along the way, Lyra and Yona faced many challenges together, and Lyra's unwavering loyalty to Yona grew deeper with each passing day.
Some fanfics also explore the idea of a 'what if' scenario where Hak didn't grow up with Yona. How would they meet and fall in love in a completely different context? This allows for a lot of creative storytelling as the writers can re - imagine their first encounters and the development of their relationship from scratch.
One idea could be an OC who is a healer and gets captured by the Akatsuki. They initially plan to use her powers but end up having a change of heart as she shows kindness. Another could be an OC with a special ability to track the Akatsuki's targets and she teams up with them reluctantly. Or an OC who is from a hidden village and has a vendetta against the Akatsuki but falls in love with one of the members, creating an internal conflict.
Another plot idea is a story where the Red Dragon has to face a new and powerful enemy alone, without Yona and the other dragons by his side. It could show his individual strength and his ability to strategize in difficult situations.
You could write a fanfiction where the Akatsuki organization from Naruto has some sort of connection to the events in Akatsuki no Yona. For instance, the Akatsuki are after a powerful artifact in Yona's world that they believe will give them ultimate power in their own world. Yona and her friends then have to stop them while also dealing with their own internal political and social issues in their kingdom.
Another interesting crossover could be with 'Sailor Moon'. The Sailor Scouts could join forces with Yona's gang. Yona could learn about the power of friendship and magic from the Scouts, and they could work together to protect both their worlds from a new, menacing threat.
One idea for an Akatsuki no Yona pregnancy fanfic could be exploring how Yona copes with the physical and emotional changes during pregnancy. She's a strong character, so seeing her deal with the new challenges while still being a leader would be interesting.
An OC with a special power related to nature could join Yona's journey. For example, the OC can communicate with animals and help the group in various situations like finding hidden paths or warning about approaching danger. This would add an interesting dynamic to the group as they face different challenges. The OC could also have a backstory about how they discovered their power and why they decided to use it to help others.
Maybe a male OC who is a healer. He joins Yona's group and uses his unique healing abilities to help the team in various situations. He also has a crush on one of the female characters in the group, which adds some romantic sub - plot.