As I don't know the 'lucy star story', I can't determine who the main character is. But based on the name, it seems that Lucy might be the main character.
Lucy could be developed by showing her struggle to control the more powerful star dresses. For example, she might initially be overwhelmed by the strength of a new star dress but gradually learn to master it through training and self - discovery. This would show her growth as a mage.
In many fanfictions about the Grand Magic Games, Lucy's Star Dress is often a key element. It can be depicted in different ways. Some fanfictions might show her using the Star Dress to gain an edge in battles during the Games. For example, she could combine different constellations' powers within the Star Dress to create unique and powerful attacks that surprise her opponents.
One popular star dress fanfiction could be 'Lucy's Starry Adventures'. In this fic, Lucy uses her star dress in really creative ways to battle villains. Another could be 'Star Dress Magic Unleashed' where the author explores different powers of the star dress that Lucy uses. There might also be 'Lucy and the Star Dress Realm' which dives into the origin of the star dress and how Lucy came to master using it.
A fanfiction idea could be that Lucy's Celestial Spirit King Star Dress is coveted by a rival guild. They try to steal it from her during a grand magic tournament. Lucy, along with Natsu and the rest of Fairy Tail, has to protect the dress while also competing in the tournament. This leads to intense battles and strategic maneuvers, and in the end, Lucy realizes the true potential of the star dress not just as a power boost, but as a symbol of her connection to the celestial spirits.