It could also be a strategic move. Gringotts is a highly secure place. By buying it, Harry could ensure the safety of important magical artifacts or secrets. He could use it as a base to store and protect things that are crucial for the future of the wizarding world, like ancient spells or prophecies.
Goblins are essential characters in Gringotts - related fanfiction. They are the ones who run the bank and have their own set of rules and personalities. Some fanfictions might introduce new goblin characters with unique traits. And then there are the dark wizards or witches who could be trying to break into Gringotts for their own nefarious purposes, like Bellatrix Lestrange who had an interest in the Lestrange vault.
A plot twist might be that the goblins turn against Harry midway through his visit. They could be influenced by a dark wizard who has a grudge against Harry. This would completely change the course of his visit and force him to find a new way to achieve his goals in Gringotts. Another possibility is that the vault contains a magical item that has a mind of its own and tries to possess Harry when he touches it.
Perhaps they refused to give Harry access to his family's vault when he really needed it. This could be because of some bureaucratic red tape or because they were trying to hide something in the vault. Such an action would make Harry angry and motivated to get back at them.
Because it can rewrite the story. Fanfiction allows authors to take the basic setting of Harry going to Gringotts and turn it into something completely different. For instance, they can make it so that instead of just retrieving money, he is on a quest to find a magical key that unlocks a powerful spell.
In some fanfictions, the goblins are portrayed as shrewd and business - minded. They are very protective of their bank and the treasures within. They are often shown as not easily trusting wizards, especially when it comes to matters of the vaults.
Perhaps the fanfiction focuses on a heist at Gringotts to steal this special weapon. The characters might face all kinds of obstacles, like the complex security systems in Gringotts, which include things like the dragon guarding the vaults and the magical spells protecting the treasures. They would need to use their unique skills and knowledge of magic to get past these barriers.
There could also be a plot where Gringotts is under some sort of dark magic threat. The characters have to figure out how to save the bank and all the treasures within. This might involve uncovering secrets about the bank's foundation and using ancient magic to counter the threat. Along the way, they could learn more about the history of the wizarding world and the role Gringotts has played in it.
You can develop characters in this type of fanfiction by giving them unique motives related to the vault. For example, if a character is a goblin, their motive could be to protect the integrity of the vault at all costs. For a wizard character, it might be greed for the treasures in the vault. By exploring these motives, their personalities can be fleshed out.