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.
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.
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.
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.
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.
One great 'Star vs the Forces of Evil Marco fanfiction' is 'Marco's New Adventure'. It takes Marco on a journey through a parallel dimension where he has to use his wits and skills in a completely different way than in the original series.
Perhaps Marco is injured while trying to protect Star from an assassin sent by one of the villains in the series. This could lead to a story where Star blames herself for his injury and has to overcome her guilt while also taking care of Marco and finding a way to stop the assassin from coming back for another attempt.
Yes, there are some great ones out there. You can start by looking on popular fanfiction websites like Archive of Our Own. There, you'll likely find a variety of stories exploring the relationship between Tom and Marco. Some might focus on their friendship evolving into something more, while others could be set in alternate universes where their relationship takes different twists and turns.
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.