Maybe it's an ancient and powerful dragon that has been in hiding for a long time. This dragon decides to raise Natsu for some prophecy - related reasons. It could be that the dragon believes Natsu is the key to saving the dragon or human world from a great danger.
One common theme is identity. The dragonborn often struggles to find their place between the dragon world and the human world (if there is any human connection in the story).
Another theme is the idea of the unknown. Raising a dragon in the wizarding world is not a common thing, so there's a lot of exploration into the unknown. Harry has to figure out how to care for the dragon, what it eats, and how to train it. This exploration of the unknown also extends to the magic that might be associated with the dragon.
In this fanfiction, it might start with Jaune as a baby being found by the dragon. As he grows, the dragon imparts its wisdom on him, like ancient languages or secrets of the land. Jaune could also have a special bond with the dragon that allows them to communicate telepathically. His journey could be about finding his place in the world, whether it's among the dragons or with the humans. He might also discover that he has a special mission, something that only he, with his unique upbringing, can achieve.
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.
One possible plot could be that Natsu, while being raised by the female dragon, learns unique dragon magic that is different from what he would have learned otherwise. He might develop a special bond with the dragon, like a mother - son relationship, and the story could follow his adventures as he uses the skills taught by the dragon to protect the dragon's lair or the surrounding area from threats.
One possible main theme could be the bond between different species. Here, it's between Lucy, a human, and the dragon. It shows that despite their differences, they can form a strong family - like relationship.
In this fanfic, there are so many possibilities. The dragon could have its own motives for raising Lucy. Maybe it sees something special in her. The story could show how Lucy's character develops under the influence of the dragon. She might become more fearless or gain some unique abilities. There could be a conflict when the outside world discovers that a human has been raised by a dragon, and they might try to separate them or use Lucy for their own purposes.
There's also 'Raised by Dragons: A Dragonborn's Journey'. In this fanfiction, the dragonborn faces many challenges as they are raised by dragons. They have to adapt to the dragon's way of life, which includes learning to fly, hunt in a different way, and communicate with other dragon species. It's a really exciting read as it shows the transformation of the character from a vulnerable being to a powerful dragonborn.