Yes, there might be some out there. You can start by checking fanfiction platforms like FanFiction.net or Archive of Our Own. Just search for 'natsu and hisui secretly married' and you'll likely find several stories. However, the quality can vary greatly, so you may need to read a few to find one that suits your taste.
Sure. In some fanfictions, Naruto's secret princely status could be a part of a much larger plot. For example, his being a prince could be the key to resolving a long - standing conflict in the ninja world. Maybe his real family was royalty from a far - off land, and he has powers or responsibilities that come with that title which he was unaware of until certain events unfold.
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.
Typically, it might start with them falling in love secretly. Maybe they meet in some secluded place often and develop feelings over time. Then, they decide to get married in a private ceremony, perhaps with only a few trusted friends or family members (if any) present. After that, they would have to deal with the challenges of keeping their marriage a secret, like hiding it from their respective friends or groups they belong to.
One common theme is power. Since Natsu is a dragon, the fanfictions often explore the extent of his power and how he uses it. Another theme is identity. Natsu has to figure out who he is as a dragon in relation to his human side.
There are definitely some interesting 'natsu is a dragon' fanfictions. One aspect that some of these fanfictions explore is the mystery surrounding Natsu's dragon form. How did he become a dragon? What are the limits of his dragon powers? These questions are often delved into in these stories. Also, the way his dragon nature affects his relationships with his friends, like Lucy or Gray, can be really engaging to read about.
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.