Marco could be developed by exploring his internal conflict. As a demon, he might have new instincts that go against his human values. For example, he may struggle with the urge to be more aggressive or use dark magic when he used to rely on his wits and martial arts skills.
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 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.
One interesting plot twist could be that Marco, as a demon, has a hidden past connection to a powerful magic source in the Star vs the Forces of Evil universe. Maybe he was once a protector of it but got corrupted.
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.
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.
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.
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.
One common theme is adventure. Since they are both involved in battles against the forces of evil in the show, fanfictions often continue this trend with them going on new quests. Another theme is friendship - turned - romance. Their close friendship in the series is a great base for writers to develop a romantic relationship in fanfiction.
A good fanfiction should first and foremost stay true to Marco's character. His sense of loyalty and his growth throughout the series should be maintained. For example, if he becomes stronger, it should be a natural progression based on his experiences. Also, the fanfiction should introduce interesting challenges that can only be overcome with his newfound strength.