One idea could be exploring how Dark Marco's powers change his relationships. For example, his friends might be afraid of him at first. Another could be him using his powers to fight new, more powerful enemies in the SVTFOE universe.
I think Pony Head would be an interesting character to pair with Dark Marco. She has a bold and sassy personality that could contrast with Dark Marco's darker, more brooding nature. Also, Marco's parents could be paired with him in fanfiction. Their reactions to his transformation and how they try to help or deal with him could add an emotional and family - centered aspect to the story.
One idea could be a crossover with another popular animated show like 'Adventure Time'. Star and Marco could end up in the Land of Ooo and have to deal with the wacky creatures there while also trying to find a way back home. Another idea might be a crossover with a superhero universe. Maybe Star's magic could combine with superhero powers in unexpected ways.
One idea could be a crossover where Naruto characters end up in the SVTFOE universe. They could learn about the magic there and maybe help Star with a big threat. For example, Naruto's shadow clone jutsu could be used in a creative way to outwit an enemy in the SVTFOE world.
One idea could be that Star has a Mewberty episode in a new dimension while Marco tries to find a way to calm her down using his knowledge of her previous Mewberty forms. Another could be a story where Marco accidentally triggers Star's Mewberty transformation during a school event, and they have to deal with the chaos together while also hiding it from others.
Another plot could involve the other characters' reactions to Star's Mewberty. For example, Jackie might be a bit jealous or confused about the new dynamic between Star and Marco. And Janna could be intrigued and try to use the situation to her advantage in some comical ways.
One great 'svtfoe star and marco fanfiction' story could be those that explore their adventures in different dimensions. For example, a story where they accidentally end up in a dimension full of musical creatures and have to use their friendship and unique skills to find their way back home.
A possible plot could involve Star's internal struggle with her feelings for Marco while also dealing with her royal duties. This would add a layer of complexity as she tries to balance her personal life and her role as a princess. For example, she might have to make a choice between attending an important royal event or spending time with Marco when he needs her.
Power is also a big theme. His vampire powers bring new abilities and responsibilities. He might struggle with controlling them or using them for good. There's also the theme of secrecy. Since being a vampire is not a normal thing in the SVTFOE world, he has to keep it a secret, which adds a lot of tension to the stories.
Well, in SvTFOE Marco x Kelly fanfiction, often it explores their relationship in different ways. It might show how they interact in various situations that are not fully explored in the original series. Maybe it focuses on their shared adventures and how they grow closer through those experiences.
One popular 'SVTFOE Vampire Marco' fanfiction is 'The Vampire's Mark'. It has an interesting plot where Marco discovers his vampire side and has to deal with the challenges that come with it in the SVTFOE universe.