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.
It could be a water - based dragon. In that case, Natsu might have some water - related magic in addition to his fire magic. This would make him a very unique character as he would be able to combine two opposing elements in a way that no one else in the Fairy Tail guild could.
Gray might show up too. He could start as a bit of a rival, not believing that Natsu can really achieve great things with this Dragon God training. But as the story progresses, he could become more of a supporter. And then there's Lucy. She might be worried about Natsu during the training, and also use her celestial spirit magic to help Natsu in some unexpected ways during his training, like summoning spirits that can provide unique challenges for Natsu to overcome.
Happy, Natsu's flying cat companion, would definitely be a main character. He would be there to provide comic relief as always, but also be a source of emotional support for Natsu during his transformation into a dragon god. Erza might also be a prominent character. With her strong leadership skills and powerful magic, she could be guiding the guild during the times when Natsu is still coming to terms with his powers and the changes it brings to the guild.
In 'natsu is alpha dragon fanfiction', it's likely a story that reimagines Natsu as an alpha dragon. This could lead to all sorts of interesting plotlines. He could be involved in territorial disputes among dragons, or perhaps he has a special mission related to his alpha nature. The fanfiction could also focus on how his friends react to his new form and the changes it brings to their relationships.
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.
In many fanfictions, Natsu's character develops by becoming more responsible. As the Red Dragon Emperor, he has greater power and with it comes more responsibility. He starts to think more about the consequences of his actions and how they affect not only his friends but also the world around him.
One great fanfiction could be 'Natsu's Ascent'. In this, Natsu discovers new powers as a third - generation dragon slayer and goes on a journey to find a hidden dragon lair. It has a lot of action and great character development.
Well, in these fanfictions, a recurring theme is the conflict between Natsu's human side and his dragon side. His dragon spirit gives him great power but also comes with its own set of challenges. Some fanfictions focus on how he balances these two aspects of himself. There's also the theme of self - discovery, as Natsu learns more about his dragon spirit, he also learns more about who he really is as a person.