Maybe an old acquaintance from Natsu's past that we haven't seen much in the original Fairy Tail story. This person could be a powerful but reclusive mage who decides to help Natsu out of respect for his father or some past debt. They could offer Natsu shelter and train him in new forms of magic to help him face the challenges during his exile.
Well, they could have an interaction where Natsu accidentally sets off Juvia's temper. Maybe Natsu accidentally melts some of Juvia's precious water - made items. Juvia gets mad, but Natsu's cluelessness about how much he's upset her is kind of endearing. Eventually, Juvia forgives him when she sees how he was just being his usual, careless self.
There could be new characters introduced, like a wandering mage who also has a troubled past. This mage could understand Natsu's feelings of abandonment and they could become friends. Also, a character from a different guild that has always admired Natsu from afar might reach out to him.
One idea could be that Natsu is exiled due to a false accusation of using dark magic. He then has to journey alone to prove his innocence, encountering new friends and foes along the way. Maybe he discovers a new form of magic during his exile.
The Fairy Tail guild members who supported Natsu's exile could be antagonists. They might believe in false accusations against him or have their own motives for wanting him out of the guild, like jealousy of his power or popularity.
It could also be a story about Natsu's personal growth during his exile. Without the support of his Fairy Tail friends initially, he has to rely on his own strength and wits. He might discover new magical abilities or face challenges that test his character, and in the end, he either returns to Fairy Tail as a changed and stronger wizard or decides that his new path outside the guild is better for him.
In fanfiction, Natsu's age can vary greatly. Some authors might keep him at his canonical age from the original Fairy Tail series, which is around 17 - 18 years old at the start. However, in alternate universe fanfictions, he could be older or younger depending on the plot. For example, in a time - travel based fanfic, he could be much older if the story takes place far into the future or much younger if it goes back to his childhood days.
Yes, in a lot of Fairy Tail fanfiction, Natsu can be overpowered. Writers often like to amp up his powers for more exciting storylines. For example, they might give him new fire - based abilities or make his existing ones even stronger to defeat more powerful foes.
Maybe some unexpected allies could emerge. For instance, other dark mages who oppose Zeref's actions for their own reasons. They might see Natsu as a valuable asset or simply dislike Zeref's overbearing ways. Another potential ally could be a powerful spirit that Natsu has befriended in the past. Spirits often have unique powers that could be useful in the fight against Zeref.
There could be a mysterious old wizard who takes Natsu under his wing when he leaves Fairy Tail. This wizard might have knowledge of ancient magics that Natsu is eager to learn.
Natsu could get really angry at Lucy. He has a bit of a temper, and being tied up against his will might make him lash out at her once he gets free. Another consequence could be that the other guild members might find out and either laugh about it or scold Lucy for her actions. It could also lead to some sort of competition between them, like Natsu trying to prove he can't be tied up so easily next time.