Yes, it could be. It might offer a fresh perspective by making Harry into a super villain. This could explore darker themes and character arcs that are different from the original series, adding a whole new layer of complexity to the story.
Well, there are 'villain Harry Potter fanfics' that focus on an alternate universe where Harry is possessed by an ancient dark magic. This causes him to act against the Order of Phoenix. In some of these stories, he might even try to resurrect Voldemort in a different way. And then there are those where Harry, due to a misinterpretation of a prophecy, decides that he should be the one to rule the wizarding world with an iron fist, believing that it's his 'destiny' in a twisted way.
One great 'Harry Potter villain fanfiction' is 'The Dark Lord Ascending'. It gives a really interesting take on Voldemort's rise to power, exploring his motives and the early days of the Death Eaters in a way the original books didn't fully cover.
Voldemort could still be a potential villain even in 'Harry Potter leader fanfiction'. Maybe he returns in a different form or has some followers who are still causing trouble while Harry is trying to lead the wizarding world into a new future.
It could be a new character created by the fanfiction author. Maybe a powerful wizard who was once shunned by the wizarding community and now wants revenge by stealing magic.
A dark wizard who has merged the dark arts with Wonderland's magic could be the main villain. He might have created some sort of dark Wonderland where all the creatures are under his control, and Harry has to break this control to restore balance.
A powerful foreign dark wizard could be the main villain. He might have heard of the prophecy about Harry Potter and decided to steal him from the Dursleys. This wizard could come from a far - off land with different and more dangerous dark magic, and his motives could be to gain ultimate power in both his own land and the British wizarding world by controlling Harry.
It could be a rogue Death Eater who escaped punishment and has been biding his time. He might have a new, more insidious plan to bring back the dark ways of Voldemort.
Perhaps the main villain is actually a corrupted version of a well - known character. For example, if Hermione was somehow influenced by the dark magic associated with the runestone path and turned against Harry. It would be a very unexpected and heart - wrenching twist, as she has always been such a loyal friend. But the power of the runestone path could be so strong that it changes her fundamental nature.
Maybe a new character, a Muggle - born wizard who was shunned by both the Muggle and wizarding worlds. This character, out of bitterness, decides to combine Muggle technology and magic to create the droid army and rule over both worlds. He would see Harry Potter and his friends as the main obstacles to his grand plan.