Another possibility is a rival wizard who is jealous of Natsu's relationship with the pet dragon. This wizard thinks that having a pet dragon gives Natsu an unfair advantage. So he tries to break their bond and steal the dragon away from Natsu. He might use various spells and tricks to try and achieve this.
Maybe there are other dragon hunters in the story. These hunters see Natsu's pet dragon as a valuable trophy. They don't care about the bond between Natsu and the dragon. They just want to prove their skills by taking down such a powerful creature. So they keep chasing Natsu and his dragon throughout the fanfiction.
Well, perhaps the fanfiction could be about Natsu training his pet dragon. He has to teach it how to fly better, control its strength, and also how to be stealthy. Along the way, they make friends with other magical creatures who also help in the dragon's training. And then they face a big challenge where the dragon's new skills are put to the test.
Maybe there are other Grimm that see the pet Grimm as an outcast for being with a human. These Grimm could come after the pet and Jaune, making them the antagonists. They would try to either kill the pet and bring it back to the Grimm world or kill both Jaune and his pet to keep their world 'pure'.
Some fanfictions feature ancient evils that were sealed away long ago but are now starting to break free. These could be powerful demons or corrupted spirits. They see Lucy and Natsu as threats to their plans of world domination, so they target them. The antagonists in these cases are not only physically powerful but also very cunning, constantly devising new ways to outwit our heroes.
It could be a powerful dark wizard who sees Natsu as a threat because of his alpha status among dragon slayers. This wizard might want to eliminate Natsu to gain control over the dragon slayers or to carry out some evil plan without interference.
There might also be some dark magic - using villains who want to use Natsu's alpha dragon slayer powers for their own evil purposes. They could try to capture him or manipulate him in some way. Maybe they plan to extract his power to create a powerful weapon or something.
A group of dark fairies could be the main antagonists. They might be jealous of Natsu's power or his unique status as a fairy and try to undermine him at every turn.
It could be other villains who are jealous of Izuku's powerful dragon quirk. They might see his quirk as a threat to their own plans for domination or chaos.
There could be a powerful demon from another dimension. As Natsu becomes an adult, his magic signature becomes stronger, which attracts this demon. The demon wants to possess Natsu's power for itself and will stop at nothing to achieve this goal. It would be a formidable foe, with abilities that Natsu has never faced before.
Maybe an old enemy of Natsu's resurfaces. This enemy is jealous of Natsu's newfound happiness with Cana and tries to take Cana hostage to force Natsu to fight him. This would create a major conflict in the fanfiction.
It could be another guild that sees Natsu as a threat. They might find out about Lucy kidnapping Natsu and try to use the situation to their advantage. For example, they could try to steal Natsu's magic or get information out of him while he's in a vulnerable state. Another possible antagonist could be a character who has a grudge against Lucy. They might try to stop her from getting away with the kidnapping or use it as a way to get revenge on her.