'Yao Xiaotian' might be referring to the character 'Yao Ye' in the novel 'Battle Through the Firmament.' He was the Young Demon Emperor of the Demon Region's Demon King Palace. His true form was the Heaven Demon Phoenix. In the novel, he had formed a friendship with the main character, Xiao Yan.
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Perhaps the fanfic focuses on the origin of Natsu's dragon - like roar. It could explore some hidden power within him that he didn't know about before. The story might involve him trying to control this new - found aspect of his power. And it could also have some sub - plots about his relationships with other characters who are either in awe or a bit scared of his roar.
Obviously, Natsu would be the main character. There could also be his friends like Lucy, Gray, and Erza. They might be either supporting him during his moments when he roars like a dragon or they could be in the vicinity when it happens and react to it in different ways.