In a fanfiction, Zeno and Shin - ah could have a spiritual journey. Since Zeno has his unique immortality and Shin - ah has his dragon powers, they might enter a dream - like realm where they meet the spirits of the past dragons. There, they learn about the true origin of their powers and the history of the dragons that has been long forgotten. This knowledge could then be used to help Yona in her quest to reclaim her kingdom.
A fanfiction could be set in a parallel universe where Zeno and Shin - ah are the main characters instead of Yona. They could be on a mission to find a cure for Shin - ah's dragon powers that are slowly consuming him. Zeno, with his immortality, would stop at nothing to save his friend.
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.
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.