One of the senior and more experienced members of the guild. They have seen a lot and know how things should be done in the guild. If Natsu acts in a way that goes against their long - established ways, they would not hesitate to berate him.
One common theme is self - discovery. When Natsu meets another version of himself, he often learns new things about his own capabilities and personality. Another theme is cooperation. Since they are essentially the same person, they have to learn to work together despite their initial differences.
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.
Gray would be extremely angry. He would immediately start looking for clues as to where Natsu was taken. His ice - make magic would be on full display as he might freeze things in frustration while trying to figure out the situation.
One possible plot could be that the kidnappers are from a rival guild who want to use Natsu as leverage against Fairy Tail. Gray could then embark on a solo mission to rescue Natsu, facing various traps and enemies along the way. He might have to use his ice - make magic in creative ways to overcome obstacles.
Perhaps characters like Gray or Erza from Natsu's side could be part of the story. They might initially be shocked but then come around to support their friend. On Ultear's side, any of her former comrades or mentors could show up. They could have different reactions, some might be happy for her while others could see it as a threat to their own plans or to Ultear's future in a more negative way.
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.
Lucy is often paired with Natsu in many fanfictions. Their relationship can be explored in different ways in these fan - created stories. It could be a more romantic relationship than in the original series, or they could have a different kind of partnership, like being more equal in power or having different goals they pursue together.
There's a great piece of 'natsu meets natsu dimension travel fanfiction' where Natsu travels to a parallel dimension and meets another version of himself. This other Natsu has different magical powers. They end up teaching each other new spells and techniques while trying to find a way to close the rift between their dimensions. It's really exciting to read how they interact and grow together.
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.