Well, perhaps the Goblin King is in possession of a key that can unlock a hidden chamber in Hogwarts which contains a powerful magic that was sealed away long ago. Harry and his friends suspect the Goblin King's intentions when they notice some strange goblin activity around the school. Or, the Goblin King might be cursed, and his actions are causing unrest in the goblin community. Harry, with his knowledge of magic and friendship with Hagrid (who has some knowledge of goblins), tries to find a way to break the curse. Also, there could be a story where the Goblin King offers Harry a deal - in exchange for helping him retrieve a lost goblin treasure, he will help Harry defeat Voldemort once and for all.
One plot idea could be that the Goblin King has a secret alliance with a dark wizard that Harry and his friends have to uncover. They might find out through a series of mysterious events and clues left in Gringotts. Another could be that the Goblin King decides to rebel against the wizarding world's control over goblin - made artifacts and starts a movement for goblin rights, with Harry caught in the middle as he tries to maintain peace. A third idea is that the Goblin King discovers a powerful ancient magic that could change the balance of power in the wizarding world and Harry has to either stop him from misusing it or convince him to use it for good.
A great plot could be that the goblin is a time - traveler. It has come from a future where Voldemort has won and is trying to change the past. The goblin teams up with Harry, using its unique goblin magic and knowledge of the future to give Harry an edge in the fight against the dark forces. They might have to travel to different magical locations and enlist the help of other magical creatures.
A really interesting plot could be that a goblin mate has a unique magical ability that is the key to saving Hogwarts from a new dark threat. The wizarding characters initially doubt the goblin but gradually come to realize their importance and the power of their bond.
One idea could be that the Head Goblin discovers a long - lost magical artifact hidden deep within the vaults of Gringotts. This artifact has the power to change the balance of magic in the wizarding world. Another could be that the Head Goblin forms an unlikely alliance with Harry Potter to protect Gringotts from a dark wizard who wants to steal all the gold and magical items stored there. Or perhaps the Head Goblin has a secret past related to Harry's family, and this connection unfolds throughout the fanfiction.
One idea could be that Harry, as king, uses his magical knowledge to create a new system of governance in the wizarding world. He could establish laws that ensure fair treatment of all magical creatures, for example, giving house - elves proper wages and rights. Another plot could involve him facing challenges from traditional pure - blood families who oppose his more inclusive style of ruling.
A really interesting plot could be that the Beast King discovers a hidden power within himself that is related to an ancient prophecy. This power could be the key to saving the wizarding world from an impending doom. Harry Potter and his gang then help the Beast King to understand and control this power.
In some goblin debt fanfics, the plot might revolve around how the goblin debt affects the wizarding economy. For example, if Harry's debt is so large that it starts to influence the value of Galleons. The goblins could use this debt as leverage to gain more control over Gringotts' operations in relation to the wizarding world. Harry then has to find a way to break free from this economic stranglehold.
One idea could be that the main character, perhaps Harry himself, befriends a goblin who has a secret knowledge about a hidden magical artifact. This could lead them on an adventure to find it before the wrong hands do. Another plot could involve a goblin friend helping Harry understand the complex and often unfair goblin - wizard relations in the wizarding world. Maybe the goblin has a personal vendetta against some wizards who have wronged his family and Harry gets involved in helping him seek justice.
Perhaps a plot where the wizard king is from a different magical realm and comes to the Harry Potter universe. He has to adjust to the different magical customs here while also trying to solve a mystery that involves a powerful magical artifact that was lost long ago. This could involve him teaming up with Harry and his friends and teaching them some new magical techniques from his own realm.
One idea could be that Harry discovers he has the power to control monsters deep in the Forbidden Forest. He uses this new ability to protect Hogwarts from a dark wizard who is summoning dangerous creatures.
One idea could be that the Fairy King has a secret alliance with Dumbledore. They work together to protect a powerful magical artifact that Voldemort is also after. Another could be the Fairy King taking Harry Potter under his wing to teach him unique forms of magic not taught at Hogwarts.