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.
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.
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.
One common theme is the exploration of their different natures. Marco is often seen as the more normal one compared to Eclipsa's otherworldly and powerful self. Another theme is friendship. They come from different backgrounds, so how they become friends or more is a big part of these fanfictions. Also, there's the theme of discovery. Discovering each other's worlds, secrets, and powers.
A popular one is 'Marco and Janna: The Mystery of the Lost Spell'. This fanfiction delves into a mystery that only they can solve. As they work together, their feelings for each other start to surface. It's full of suspense and also some sweet interactions between the two characters.
One common theme is probably adventure. Since the original show has a lot of adventure elements, the fanfiction might continue that trend with Marco and Kelly. Another could be love and relationship growth, as fans are interested in seeing how their relationship evolves.
Another theme is self - discovery. Both Tom and Marco are on a journey to figure out who they are. In the context of their relationship, they help each other in this process. Like, Tom might help Marco embrace his more adventurous side, while Marco helps Tom control his temper.
The theme of self - discovery is also quite common. As Star and Marco are together, they learn more about themselves. Also, the idea of facing challenges as a team is prevalent. They might face an evil force or a difficult situation in the fanfic, and through working together, they overcome it. Like in some fanfics where they have to save their home from a great danger.
Fanfiction.net is also a great place to find such stories. There are many authors who post their works about Star and Marco there. Just use the search function and type in relevant keywords like 'Star vs. The Forces of Evil Star and Marco' to find what you're looking for.
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.