Obviously, Voldemort could still be the main antagonist. He would be intrigued and threatened by Harry's new power, the lightsaber. Also, some Death Eaters might team up with dark wizards from other lands who are also after the power of the lightsaber.
The pure - blood supremacists could be the main antagonists. They might oppose Harry's ideas of a more inclusive and democratic political system as they believe in the superiority of pure - blood wizards.
It could be the Death Eaters who see Harry's new - found mastery of darkness as a threat to their own power and ideology. They might try to either recruit him or destroy him. Voldemort himself could be an antagonist, feeling that Harry is encroaching on his territory as the most powerful dark wizard.
The Ministry of Magic could also be antagonists. They are often very strict about magic use and might not approve of Harry's exploration of wandless magic. They could try to stop him, fearing that it would disrupt the established order of magic that is mostly wand - based. The Ministry officials might see Harry as a troublemaker and try to put him under surveillance or even arrest him if they think he's getting too powerful in wandless magic.
There could be a powerful dark entity that has been awakened by Harry Potter's newfound mastery of fire and darkness. This entity could be an ancient evil that was once sealed away and now wants to use Harry for its own dark purposes or simply destroy him as he is a potential obstacle.
Well, perhaps Harry discovers an ancient magic that allows him to wield a lightsaber. He could use it to fight Voldemort in a more unique way, combining the magic of the wizarding world with the power of the lightsaber. Maybe he has to go on a quest to learn how to master it fully.
It could be a dark wizard who wants to get his hands on the lightsaber for its power. He might see it as a new tool to use in his evil plans, like using it to break into Gringotts or to fight off any who oppose him.
I think Hermione would be the most likely. She's very intelligent and a quick learner. She would study all the available information about lightsabers, probably find some ancient texts in the library about similar weapons. With her determination and knowledge - seeking nature, she'd be able to master the basic techniques of lightsaber handling in no time.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.
One possible antagonist could be a corrupt wizard in a high - ranking position in the Ministry of Magic's financial department. This wizard might be trying to cover up his misdeeds and would see Harry as a threat to his scheme.
Well, in theory, it could. Fanfics often like to blend different fictional universes. However, in the official Harry Potter lore, there are no lightsabers. But in a fanfic, an author could come up with some creative magic or a cross - universe event that brings lightsabers into the wizarding world. Maybe a powerful wizard discovers a way to create a weapon similar to a lightsaber using magic and some rare materials.