In such a fanfiction, perhaps Harry Potter could use his magic to find an unclaimed land. He might then use spells like 'accio' to gather resources for building infrastructure. He could also use magic to educate the people, like creating self - teaching spell books.
Well, Harry Potter might build a country based on the principles of Hogwarts. For example, there could be houses similar to Gryffindor, Hufflepuff etc. for different groups of people. He could use his fame to attract wizards and witches from all over the world to join his new country. And he would definitely need to create a magical defense system around the country to protect it from dark wizards.
One possible plot could be that Harry Potter, after defeating Voldemort, realizes that the wizarding world still needs a new kind of society. So he starts from scratch. He uses his knowledge of magic and his experiences at Hogwarts. He might use the Room of Requirement as an inspiration to create buildings that can change according to the needs of the people. He would also need to establish laws, which could be a mix of traditional wizarding laws and some new ones he creates based on equality and justice for all magic - users.