I'm sorry, I haven't read 'king zeno a novel' specifically, so I can't provide much detail. You could try checking book review websites or asking in a literature - focused forum.
One common theme is adventure. Since both Lucy and Zeno can be characters in action - packed stories, their fanfiction often involves them going on quests together.
Perhaps Gohan meets Zeno during a multiverse tournament. Gohan, being the smart and powerful warrior he is, could start a conversation with Zeno about the importance of protecting all universes, not just for the sake of power but for the life forms in them. Zeno might be intrigued and start to see the universes in a new light.
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.
It could be someone who doesn't want Gohan and Zeno to form an alliance. Maybe a rogue god from another universe who fears that if Gohan influences Zeno, his own plans for universal domination will be thwarted.
One popular 'zatch bell zeno fanfiction' might be those that explore the deeper backstories of the characters. For example, a story could focus on Zeno's early life and how he came to be the way he is in the original series.
Social media groups related to 'Zatch Bell' can also be a good source. For instance, on Facebook or Reddit groups dedicated to the series, fans might post links to their 'zatch bell zeno fanfiction' or talk about their favorite ones.