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.
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.
It could be Zeref. Since he has a complex relationship with Natsu in the original story, he might manipulate things behind the scenes to make the Fairy Tail guild betray Natsu. Maybe he wants to isolate Natsu for his own evil plans.
Perhaps an old enemy of Fairy Tail who sees Natsu as a new threat. They might target Natsu specifically to prevent him from strengthening the guild. This antagonist could have a long - standing grudge against Fairy Tail and see Natsu as the new hope for the guild, so they try to eliminate him before he can fully join.
A powerful demon from another dimension could be the main villain. This demon has noticed Natsu's growing power and sees him as a threat. The demon has the ability to create illusions and false memories, which could be used to drive a wedge between Natsu and the Fairy Tail guild. He works in secret, slowly weakening Natsu's mental state before making a big move.
Erza would likely have a more serious take on it. If the fanfiction adheres to the values of Fairy Tail and portrays Natsu in a positive light that aligns with his character in the guild, she would probably approve. However, if it contains any inappropriate or un - Fairy Tail - like behavior, she would surely be against it.
It could be about Natsu having left Fairy Tail for some reason and then reuniting with the guild in a very unexpected way. Maybe he has new powers or a different outlook on life that causes some friction and also some amazing moments of reunion and growth within the guild.
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.
I'm not sure who 'natsu' specifically is as the founder of Fairy Tail fanfiction. There could be many people creating fanfiction and using the name Natsu in their works. It might be a self - given title by an individual within the fan community.
In some fanfictions, Natsu as the 1st master of Fairy Tail might be depicted as a powerful and visionary leader. He could be shown as someone who laid the foundation for the guild's values of friendship and strength.
Well, without more context, 'her' could be any number of characters. It might be a new character created for the fanfiction. Natsu might want her to join Fairy Tail because he sees potential in her magic abilities. Fairy Tail is all about accepting people with different powers and making them part of the family.