The antagonists could also be important characters in this fanfiction. Voldemort might be long gone, but there could be new dark wizards or witches who rise up against Harry's plan for world domination. Maybe some pure - blood supremacists who don't want to see the world change in the way Harry envisions. These characters would create conflict and drive the plot forward.
In addition to the trio, some other characters from the original series could become key players. Neville Longbottom, for instance, could be a strong ally to Harry. He has proven his courage and loyalty in the past. And characters from different wizarding schools around the world might also be introduced. They could either support or oppose Harry's attempts to take over the world, adding more complexity to the story.
Obviously, Harry Potter himself would be the central character. His friends like Hermione and Ron would probably also play important roles. They might be his advisors or the ones helping him carry out his plans. Dumbledore could also be a significant figure, even if only in spirit or through flashbacks, guiding Harry's actions as he tries to take over the world.