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.
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 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.
One possible scenario could be that Natsu's dragon slayer magic goes out of control, causing him to gradually transform into a dragon. This might be due to some powerful dark magic interfering with his natural magic flow.
In addition to Natsu, Wendy could be a main character in the 'natsu is a dragon god fanfic'. Since she also has dragon - related powers, she could have a special connection with Natsu in this new situation. Maybe she helps him understand more about his dragon god powers. And Makarov, as the guild master of Fairy Tail, would also be a significant character. He would be concerned about how Natsu's new status affects the guild and the magical world as a whole.
Potentially, we could see some celestial dragon characters. They might be ancient and powerful beings. Some could be friendly towards Natsu, seeing him as a new hope for their kind. Others might be more hostile, seeing him as an intruder or a threat to their traditional ways. There could also be other mages from different guilds who are either curious or envious of Natsu's situation with the celestial dragons.
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.