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.
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.
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.
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.
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.
Erza could also be a possibility in some fanfictions. She is strong and independent, and Natsu respects her a great deal. A relationship between them would be full of interesting dynamics, like how they would handle their different leadership styles when they are together, and how their relationship would affect the guild as a whole.
A character from a different guild who is on the run from some dark forces. Natsu could offer protection and in the process, this character could introduce new fighting styles and magic types to Fairy Tail. Also, a magical creature that can talk and has the power to sense hidden magic could be added. Natsu could befriend this creature and it could help him in his quests.
He could meet powerful mages from other guilds. These mages might have different styles of magic that Natsu can learn from.
Well, he could meet some old friends or foes. Maybe he runs into an old enemy who has also been training and they have a rematch which pushes Natsu to new limits. Or he meets an old friend who gives him some valuable advice on how to control his magic better during this trip.
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.
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.