Well, you could write a fanfiction where the reader is a spirit - like being that only Yona can see. This being gives Yona advice and support in her toughest times. Or perhaps the reader is a lost warrior who has forgotten their past, and Yona's group helps them regain their memories while they, in turn, help the group with their combat skills.
First, decide on the role of the reader in the story. Are they a friend, a helper, or something more? Then, think about the setting. Will it be in one of the existing locations in the 'Yona of the Dawn' world or a new place? After that, just start writing the first encounter between the reader and Yona or her group. For example, you could start with the reader saving one of Yona's friends from a dangerous situation.
One common theme is adventure. Since the original story has a lot of adventure elements, fanfics often continue this. Yona and Hak go on quests together, facing enemies and exploring new lands, which strengthens their bond.
One popular 'Yona of the Dawn Yona x Hak fanfic' is 'The Journey Ahead' which beautifully explores their relationship as they face various challenges together. It has great character development and a lot of sweet moments between Yona and Hak.
One more theme could be the exploration of the fantasy world. Writers might create new areas in the 'yona of the dawn' world, or expand on the magic and mythology that already exists in the original story. This can make for some really interesting and imaginative fanfictions.
Yes. The Yona of the Dawn manga has ended. Although it might be sad for fans, it left a memorable impression with its engaging plot and characters throughout its run.