One interesting idea could be exploring a darker side of Marco's character. Maybe he has some hidden powers or motives that are revealed in the fanfiction. Another idea could be his relationships with other characters in this 'evil' scenario. For example, how does his friendship with Star change when he takes on this new, darker persona.
In this type of fanfiction, an interesting aspect could be the origin story of 'Evil Marco'. It could be that he was influenced by some dark magic or had a bad experience that turned him this way. The fanfiction might also explore how the overall plot of 'Star vs the Forces of Evil' changes with an evil Marco in the mix. His actions could lead to different alliances and conflicts. For instance, he might form an unexpected alliance with some of the show's villains, or his actions could cause a rift between the main characters who are usually on the same side.
One great fanfiction could be about Marco's journey to master a new and powerful spell. In this story, he might face various challenges that test his strength and determination. He could encounter unique enemies that force him to dig deep into his magical abilities and his own willpower. This would show his growth as a character from the series.
There are many great fanfictions about Star and Marco. Some might focus on their adventures that could have happened off - screen in the show. For example, a fanfiction could explore a parallel universe where they have different powers but still their strong bond remains.
One common theme is friendship. Since Marco has many friends in the show, fanfictions often explore the depths of his friendships, like how he supports his friends and how they support him in return.
Sadly, I don't have a particular 'Star vs the Forces of Evil Evil Marco Fanfiction' to recommend directly. However, when you search for it, look for ones with high ratings and a lot of reviews. Those are usually the better - written and more engaging ones. Also, some fanfiction communities on social media might be able to give you some good suggestions.
There could be a fanfiction that puts Evil Star in a situation where she has to face her own good side. Maybe she gets hit by a spell that makes her see the consequences of her evil actions. This could lead to an internal struggle within her, and the story could explore whether she would change or double down on her evil ways. It would be really interesting to see how the writer develops her character in such a scenario.
One idea could be creating an OC who is a hybrid of different magical species in the 'Star vs the Forces of Evil' universe. For example, part Mewman and part monster. This OC could struggle with acceptance from both sides. Another idea is an OC who has a unique magic ability that no one has seen before, like the power to control time on a small scale. This could lead to all sorts of adventures and challenges as they try to master it without causing chaos.
Another interesting plot could be Meteora forming an unlikely alliance. For example, she teams up with a group of misfits who are also on a quest for something. Together, they face various challenges and in the process, Meteora learns about friendship and trust.
Another possibility is a crossover with 'Avatar: The Last Airbender'. Star could end up in the Avatar world and learn about bending. She might team up with Aang and his friends to fight against Fire Nation soldiers who have somehow gotten a hold of some of Star's magic spells and are using them for evil.
One idea could be exploring an alternate universe where Star and Marco's roles are reversed. Another could be a fanfic that focuses on the backstory of a minor character like Pony Head. Or perhaps a story where the forces of evil have a more sympathetic origin.
One common theme is identity. Since Marco is now a female, exploring how this new identity affects her relationships and her role in the story is often seen. For example, how does she interact with Star differently now? Another theme is power. Some fanfictions give Fem Marco new powers, and the exploration of how she uses and controls these powers is a big part of the story.