In fanfiction, Natsu's Dragon Form could be developed to have a symbiotic relationship with other characters. For example, he could combine his power with Lucy's Celestial Spirit Magic in a way that enhances both of their abilities while in his Dragon Form. This would create unique and interesting battle strategies.
One idea could be that in fanfiction, Natsu's Dragon Form could have unique fire abilities. For example, he might be able to create different shapes of fire, like a dragon made entirely of fire that can attack enemies on its own. Another could be that his Dragon Form has a connection to an ancient magic that gives him the power to heal other dragons or dragon slayers.
Natsu's character development in this fanfiction might involve him becoming more responsible. With his end form, he has greater power and with that comes the need to use it wisely. He could start to think more about the consequences of his actions. For example, he might not rush into battles as blindly as he used to. Also, his emotional growth could be a part of it. He could become more in tune with his feelings towards his friends, especially Lucy. His loyalty to the guild could also deepen as he realizes his new form can be a great asset to protect Fairy Tail.
Gray would also be important. There could be a rivalry - turned - friendship aspect where Gray, who has his own ice - making powers, and Natsu, as a dragon, have to learn to work together. Maybe they compete in a friendly way to see whose powers are more useful in different situations.
One interesting concept could be Natsu's enhanced power levels. In end form, he might have new and more powerful fire-based attacks. For example, he could create a massive inferno that not only burns but also has some kind of special effect like nullifying dark magic in the fanfiction world. Another concept could be his appearance. Maybe he has a more mature and fierce look, with his hair changing color slightly or his eyes glowing more intensely. Also, his relationships with other characters might be developed further. His bond with Lucy could become even stronger, and they could have new adventures together where his end form is crucial for their success.
One interesting plotline could be Natsu discovering new dragon powers that are unique to him. This could lead to him having to learn how to control these powers and facing challenges within the Fairy Tail guild as others may be wary or envious of his new abilities.
One interesting concept could be exploring Natsu's new powers and abilities in the Dragon Force state in more depth. Maybe he discovers hidden techniques or ways to control the power better.
Often, it could be because of a false accusation. Maybe someone framed Natsu for a crime he didn't commit, like stealing a powerful magical artifact from the guild. So, to maintain peace and order, the guild leaders had no choice but to banish him.
One great fanfiction that delves into Natsu's memories might be titled 'Natsu's Hidden Past'. In it, the author could explore his early days in Fairy Tail, his relationship with Igneel, and how those experiences shaped the person he is today. It might show how he learned to control his dragon slayer magic and the friendships he formed along the way.