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.
Seilah is also a main character in this context. She has her own set of powers and a rather complex personality in the Fairy Tail universe, which can be further explored in fanfiction.
Obviously, Natsu is a very popular character as it's in the title. He is often the center of many fanfictions. Lucy is also quite popular as she has a close relationship with Natsu in the original series. Their dynamic often plays a big part in fanfictions.
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.
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.
Often, it could be because of a false accusation. Maybe someone framed Natsu for a crime he didn't commit, like stealing a powerful magical artifact from the guild. So, to maintain peace and order, the guild leaders had no choice but to banish him.
Gray is another character. His ice - magic can complement Natsu's fire - magic. They might team up to fight enemies or overcome obstacles in the way of saving Fairy Tail. Sometimes they bicker while doing so, which adds some humor to the story.
Gray could be one. Since he and Natsu have a long - standing rivalry, Gray might try to bring Natsu back to Fairy Tail. He'd be conflicted between his annoyance with Natsu and his concern for his former guildmate.