Well, one factor is a powerful and confident version of Harry. For example, if he has some new and extraordinary magical abilities that set him apart. Like in some fics where he can control all the elements, that's really badass. Another aspect could be the way he stands up to the villains. If he doesn't wait for them to make the first move but actively hunts them down.
One great 'badass powerful' Harry Potter fanfiction is 'The Lightning in the Night'. In this fic, Harry discovers some ancient magic that makes him extremely powerful and he uses it to take on the Death Eaters in a very cool way. He's not the meek boy we know from the original series but a force to be reckoned with.
One great 'Harry Potter Badass Harry' fanfiction is 'The Boy-Who-Lived-Again'. In this fic, Harry returns with a newfound confidence and power. He's not the meek boy we saw in some parts of the original series. He quickly masters advanced magic and outsmarts his enemies in a really cool way.
One great 'fem Harry Potter badass fanfiction' is 'The Girl Who Lived'. In this story, the female version of Harry is portrayed as extremely tough and intelligent. She faces the challenges of the wizarding world with a unique perspective and unwavering courage.
It can be really good. Fanfictions often explore different aspects of the original story. In this one, seeing Harry as a 'badass' in Slytherin might lead to interesting character developments and interactions that we don't see in the official books. For example, it could show how he navigates the Slytherin house politics and still maintains his heroic qualities in a different setting.
One key factor is the world - building. In a 'badass Harry Potter fanfiction', the author might expand on the wizarding world in ways that J.K. Rowling didn't. For instance, exploring new magical lands or hidden parts of Hogwarts. If the story also has a darker or more complex tone, it can be more appealing. Think about a fanfiction where the political situation in the wizarding world is much more convoluted and Harry has to navigate through it while being a 'badass' leader or rebel. Also, when the fanfiction challenges the established norms of the original series. For example, if it changes the house dynamics so that Gryffindors are not always the 'good guys' and Slytherins have more depth than just being the 'bad guys'. This kind of subversion can make a 'badass Harry Potter fanfiction' really stand out.
One popular 'badass Harry Potter fanfiction' is 'The Black Family Values'. It gives a different take on the Black family and makes Harry a more assertive and powerful character. Another could be 'A Black Comedy by Saffron Bryant' which has a really unique plot and a very 'badass' version of Harry.
One unique feature is the combination of different universes or concepts. For example, if it's a crossover with a superhero universe, you get to see how magic and superpowers interact. It's like a mash - up of two different worlds of fantasy.
One common theme is empowerment. The female characters are often shown as being just as powerful, if not more so, than their male counterparts in the original series. For example, they might be better at magic or more strategic in dealing with threats.
It's appealing because it gives a new perspective. In the original series, female characters often played supporting roles. In 'badass female Harry Potter fanfiction', they take center stage and show that they can be just as powerful and heroic as male characters. It allows for more diverse interpretations of the wizarding world.