In this fanfiction, Harry Potter might be facing a new situation in the Muggle world. Buying a car could be a sign of his further integration into Muggle society. Maybe he uses his wizard money to buy a really cool and unique car, like a classic convertible.
Ron might be there too. He could use his skills in handling magical creatures to populate the island with friendly ones. And he could also help with fishing in the surrounding waters using magic, providing food for those on the island.
Yes, there might be. The Harry Potter fandom is vast, and fanfiction writers come up with all sorts of creative and unique ideas. You can try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net. There could be a story where Harry Potter is involved in buying a marriage contract in some alternate universe or as part of a very different plotline created by the fanfiction author.
One challenge would be the goblins. They are very protective of Gringotts and would not easily give up control. They have their own laws and traditions regarding ownership. Harry would have to deal with their resistance and find a way to convince them or negotiate with them.
Well, perhaps Harry, tired of the wizarding world's chaos, decides to buy an island for some peace. He could use magic to build a unique, hidden mansion there. Maybe he'd invite his closest friends like Hermione and Ron to visit, and they'd have magical adventures on the island that are different from those in Hogwarts.
Well, he might choose a house in a wizarding village. Maybe it's a small cottage with a charmed garden full of magical plants. He could use his Triwizard Tournament winnings to pay for it.
Maybe Harry Potter, after all his adventures, decides he needs a peaceful place. He uses his wizarding money to buy an island. He might turn it into a magical sanctuary for magical creatures, away from the prying eyes of Muggles.
He might want a place of his own, away from the chaos of Hogwarts and the wizarding world's constant threats. A building could be his private sanctuary where he can relax and practice magic without being bothered.
He could use the money left to him by his parents. They were wealthy and it's likely there was a large inheritance waiting for him. Plus, he had his own earnings from winning the Triwizard Tournament.
One possibility is that Harry is exploring the Muggle world more and decides to buy Grunnings out of curiosity. Maybe he wants to understand his uncle Vernon's world better. He could use magic in secret to improve the business operations and make it a more efficient place, all while keeping the Muggles there oblivious to the magic involved.