A common theme is also family. Given their complex family history in the comics and movies, fanfic writers often delve deeper into that. They might explore their relationship with Magneto as their father, or how they support each other as siblings. And of course, there are romances. Some fanfics pair Scarlet Witch with Vision or other characters, and explore how Quicksilver reacts to that as her brother.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are a ton of stories there about Quicksilver and Scarlet Witch. Just search for their names in the search bar and you'll get a whole list of fics. Another place could be FanFiction.net, though AO3 generally has a wider range of more diverse stories in my experience.
It's not straightforward to say she was evil. The Scarlet Witch's behavior in the comics was influenced by many factors, and her intentions weren't always malicious. Sometimes her powers caused unintended consequences that were seen as negative.
Scarlet Witch is indeed a mutant in the comic universe. Her mutant powers give her unique capabilities and play a significant role in many storylines. However, the extent and nature of her powers have evolved and changed over time.
The Scarlet Witch's character in the comics is complex. She has moments of both good and bad, depending on the storyline and her motivations. So, it's not a simple yes or no answer to whether she's evil.