Well, a great fanfiction idea could be that Darth Sidious discovers the existence of the wizarding world through some dark magic connection. He decides to conquer it, thinking it would be easy prey. But he underestimates the power of the Hogwarts students and their loyalty to each other. Harry Potter would be at the forefront of the resistance, using his skills and the support of his friends like Hermione and Ron.
If it was a fanfiction like that, it would completely distort the perception of Harry Potter. Harry is a symbol of hope and light in his own universe, but if he was like Darth Vader, he'd be seen as a much darker and more menacing figure. As for Darth Vader, his iconic status as a Sith Lord might be diluted if his traits were randomly imposed on Harry Potter. It would be a very odd mash - up.
A fanfiction could explore the idea of Darth Sidious being somehow trapped in the Harry Potter universe. He'd be completely out of his element, yet still a formidable threat. Harry Potter, with his knowledge of the wizarding world and his courage, would be the one to stand up to Sidious. This could lead to a unique mentorship situation where Dumbledore tries to understand the nature of the Force from Sidious in order to find a way to defeat him, while also teaching Harry how to resist the Sith's temptations.
First, you need to do your research on both universes. Know the rules of magic in Harry Potter and the nature of the Force in Star Wars. Then, think of a unique premise for how the two worlds will collide. For example, a magical accident that transports Darth Sion to Hogwarts.
Fanfiction.net is also a popular place to look. Just use the search bar and type in 'Darth Harry' and it should show you the relevant stories. However, the quality can vary, so you might need to skim through a few to find the really good ones.
There are 'Darth Harry' fanfictions that focus on his relationships. For example, how his friends from the Harry Potter world react to his new, darker self. Some stories might have Hermione trying to bring him back from the dark side, while others could show Ron being more afraid of him. These relationship dynamics add depth to the fanfiction and make it more interesting to read.
In the fanfiction, Darth Revan's training of Harry Potter would be a blend of two different training regimens. Revan would begin with mental training. He would teach Harry to clear his mind, much like a Jedi does, but with a Sith - like focus. This would help Harry control his magic more precisely. Next, he would focus on Harry's combat skills. He would show Harry how to use his wand more like a weapon, not just a tool for casting spells. For instance, using it to block physical attacks as well as magical ones. He would also introduce Harry to the concept of using the environment in combat, like using objects around him to enhance his spells or create distractions. And he would constantly push Harry to his limits, believing that only through extreme challenges can one truly grow in power.
One common theme is the clash of different power systems. Since Harry Potter has magic and Darth Nihilus has the Force, exploring how these two interact is a popular theme. For instance, in many fics, they might test which is stronger or how they could be combined.
In a fanfiction where Harry fights Darth Sidious, it could go in many ways. Maybe Harry uses his magic skills like the Patronus charm to fend off Sidious' dark side Force attacks. Since Harry has shown great courage and quick thinking, he might be able to hold his own against Sidious at first. But Sidious is extremely powerful in the Dark Side of the Force. So, it would be a tough battle and perhaps Harry would need to find some way to disrupt Sidious' connection to the Force, like using magic to create interference.
One possible plot could be that Harry discovers he has a connection to Darth Revan through some ancient magic or a prophecy. He then starts to develop powers similar to the Force, like telekinesis but in a magical way. He might use these new abilities to fight Voldemort and his Death Eaters, taking them by surprise as they've never seen such powers before.