Goku from Dragon Ball Z could be a great crossover character for Natsu. They are both very strong and have a love for fighting. Goku's ki - based attacks and Natsu's fire magic would make for some epic battles. They could also train together and learn from each other's fighting styles. Also, Alucard from Hellsing could be an interesting crossover. His vampiric powers against Natsu's magic would create a very dark and intense story.
In a crossover with 'Naruto', Naruto himself would be a great character to pair with Natsu. They both have strong personalities and are determined to protect their friends. They could learn from each other's fighting styles, with Naruto showing Natsu some of his ninja techniques and Natsu teaching Naruto about magic.
A fanfiction idea could be that in a crossover, Natsu is tired because he's been on a long - term mission in a different realm. He returns to Fairy Tail not quite himself. The guild members start to worry as he doesn't have his usual energy for pranks or challenges. As the story progresses, he slowly regains his strength through the support of his friends and the familiar environment of the guild, and in the end, he's back to his old self ready for new adventures.
Erza is frequently paired with Natsu in these fanfictions. Given her strong and leadership - like qualities, she can take charge in agent - related situations, and Natsu can be the more impulsive force. Their interactions can range from Erza scolding Natsu for his reckless actions to them having deep conversations about their responsibilities as agents. Wendy is also a character that can be paired with Natsu. Her healing magic can be crucial during dangerous agent missions, and she looks up to Natsu, which can create a sweet and protective relationship in the fanfiction.
Gray would be one of the main characters to interact with Natsu. Their friendly rivalry is well - known in Fairy Tail. Makarov, as the guild master, would also have an important role in interacting with Natsu. He might give Natsu some important tasks or just have a grandfatherly chat with him about his return.
Gray would also be important. There could be a rivalry - turned - friendship aspect where Gray, who has his own ice - making powers, and Natsu, as a dragon, have to learn to work together. Maybe they compete in a friendly way to see whose powers are more useful in different situations.
Lucy is often paired with Natsu in many fanfictions. Their relationship can be explored in different ways in these fan - created stories. It could be a more romantic relationship than in the original series, or they could have a different kind of partnership, like being more equal in power or having different goals they pursue together.
He could be less active in battles. Usually, Natsu is full of energy and jumps right into fights, but if he's tired, he might hold back or even need to be rescued by his comrades. This would be a new and interesting dynamic for the story.
One idea could be Natsu crossing over into a modern-day world. He'd be confused by all the technology but use his magic in unexpected ways, like using fire magic to power up electrical devices. Another could be a crossover with a different anime world where Natsu has to team up with new characters to fight a common enemy. For example, in a world full of mecha, he might use his fire to overheat the enemy mechas.
Gray would be shocked at first. He and Natsu have a long - standing rivalry - but also a deep friendship. He might initially be angry that Natsu would just leave like that, but then he would realize how much he'll miss him. He might also feel a sense of loss for the guild as a whole, since Natsu is such an important part of it.
Lucy could be a main character. She was always close to Natsu in the original Fairy Tail. So when Natsu returns as an 'ex' and meets the guild again, she would likely play a big role in how he is received and in the development of the story.