In a fanfiction about the return of Natsu's foster father dragon, perhaps the dragon returns with a mission. He might have learned of a great threat during his time away and has come back to enlist Natsu's help. This could involve traveling to new and dangerous lands, encountering unique magical creatures, and Natsu having to prove himself once again to his foster father. The story could explore their relationship, which may have some strain due to the long separation.
The plot could be that the dragon's return disrupts the current peace in Natsu's world. Maybe the dragon has some unfinished business with an old enemy, and now that he's back, he drags Natsu into it. Natsu, being loyal to his foster father, follows him into this new conflict. This could involve a lot of action - packed battles and some emotional moments as Natsu has to deal with the potential danger to his foster father.
In some fanfictions, Acnologia is made out to be Natsu's father. But this is not part of the original canon of the Fairy Tail series. Fanfiction writers often like to create such unexpected family relationships to add new twists to the story.
Yes, there are likely fanfictions out there with such a concept. Fanfiction platforms are full of creative and often wild ideas. You can check major fanfiction websites like FanFiction.net or Archive of Our Own (AO3). Just search for the relevant tags or keywords to find them.
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.
Fan Xian's adoptive father was Fan Jian. Fan Jian was portrayed as a shrewd and upright person in the Qing years. In order to protect the interests of Fan Xian and his mother, Ye Qingmei, he did not hesitate to use his own son to replace Fan Xian's identity and raise Fan Xian by the old lady's side. Fan Jian had displayed outstanding resourcefulness and strength, and he had become the Minister of Revenue and Count Sinan. His feelings for Fan Xian stemmed from his love for Ye Qingmei. He was willing to give up everything to protect Ye Qingmei and Fan Xian. Fan Jian's goal was to let Fan Xian inherit the power of the palace treasury and become a happy rich man. However, Fan Jian's father-son relationship was not cultivated before Fan Xian grew up because they had never met. Thus, the father-son relationship between Fan Xian and Fan Jian might have been built on Fan Jian's love for Ye Qingmei and his protection of Fan Xian.
Sorry, as a fan of online literature, I don't have personal taste and aesthetic views. I can only provide information and comments about these novels, but I can't comment on whether the works themselves are good or not.
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.
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.