As Lord Slytherin, Harry could form an alliance with the international wizarding community. He could represent a new face of Slytherin that is not associated with the dark past. This could lead to new exchange programs between Hogwarts and other wizarding schools. He could also form an alliance with some of the older wizarding families who have ties to Slytherin but have distanced themselves from the more extreme views. By doing so, he could bring about a more unified and progressive wizarding world.
Harry could form an alliance with the goblins. Since Slytherin was known for his cunning and business - like acumen, Harry as Lord Slytherin could use this to his advantage. He could negotiate with the goblins for better terms regarding Gringotts and perhaps form an economic alliance that benefits both the wizarding world and the goblins. Also, he might form an alliance with some of the magical creatures. Slytherin had an affinity for snakes, so Harry could use this connection to reach out to other magical creatures, building bridges between humans and non - humans in the wizarding world.