You could write a fanfiction where Gajeel enters Dragon Force during a major battle against a powerful dark guild. In this story, his transformation could turn the tide of the battle and inspire his comrades. He could also have some internal struggle during the transformation, like dealing with the overwhelming power and the fear of losing control.
First, you need to know the character well. Read the original manga or watch the anime to understand Gajeel's personality, his relationships, and how his Dragon Force is presented. Then, come up with a simple idea, like a situation where he has to use his Dragon Force in an unexpected way.
Their relationship can be developed through shared experiences. For example, they could be on a mission together where they have to rely on each other's strengths. Gajeel's physical power and Levy's intellectual power would complement each other, and this would lead to a deeper understanding and respect between them.
Levy's character could be developed by showing her growth from a sheltered princess to a strong and independent woman. For example, through her interactions with Gajeel, she might learn to be more assertive. Gajeel, on the other hand, could start as a rough - around - the - edges dragon but become more gentle and caring as he gets closer to Levy.
One interesting plot idea could be that Gajeel discovers a new type of dragon magic that only Levy can help him understand. Maybe it's a magic that involves ancient dragon runes, and Levy's knowledge of books and languages is crucial in deciphering it. As they explore this new magic together, their relationship deepens.
In some fanfictions, Natsu might gain the ability to use Dragon Force through intense training. He could be pushed to his limits by a powerful opponent or a difficult situation, forcing his body and magic to adapt and unlock this powerful form.
A common theme is power. Wendy's acquisition and control of Dragon Force powers. Another is adventure, like going on quests or exploring unknown areas related to the Dragon Force.
One plot idea could be that Levy, as the princess, has a secret power that only Gajeel, the dragon, can help her control. As they interact, they start to develop feelings for each other. Another could be that Levy is kidnapped by an evil force and Gajeel, being the powerful dragon, goes on a dangerous quest to rescue her, during which they discover a long - lost prophecy about their fates.
Typically, he has enhanced strength. His physical attacks become much more powerful, allowing him to defeat stronger opponents with ease. For example, he could punch through solid rock or metal without much effort.
One great Levy and Gajeel fanfiction is 'A Bond Beyond Words'. It beautifully explores their relationship growth from rivals to lovers. The author does an amazing job of capturing Levy's intelligence and Gajeel's rough - yet - tender side.