Good fanfiction writing often involves character development. For Star, show how her Mewberty experiences change her over time. Maybe she becomes more confident or more in touch with her magical side. For Marco, he could develop a deeper understanding of magic through his interactions with Star in her Mewberty state. The plot should be engaging. You could have a mystery related to Star's Mewberty, like an ancient prophecy that only her Mewberty form can fulfill. And don't forget about the dialogue. Make it sound like how Star and Marco would actually talk to each other.
To write a good fanfiction, start with creating a vivid setting. Since Star is from another dimension, you can play with different magical landscapes. When it comes to the Mewberty aspect, describe it in detail. How does Star look? What are the new powers she has? For the relationship between Star and Marco, show their growth. Maybe Marco is initially scared of Star's Mewberty form but gradually learns to accept and work with it. Also, add some conflict, like a villain who wants to use Star's Mewberty power for evil.