I'm not sure specifically what this 'natsu is old man fanfiction' is about as there could be many interpretations. It might be a fan - created story where a character named Natsu is depicted in some way related to an old man, perhaps in an alternate universe scenario, or a story exploring a relationship between Natsu and an old man character.
You can try looking for it on popular fanfiction websites such as Archive of Our Own or FanFiction.net. Just use the search bar and type in 'natsu is old man' to see if any relevant stories pop up.
Natsu might be initially shocked and confused. He's not the type to expect rejection, so he could be at a loss for words at first. For instance, he might just stand there with his mouth open, not believing what he just heard.
Well, in many fanfictions, Natsu might initially be in shock. He's a passionate character, so he could react with anger. He might lash out at Lisanna or the person she cheated with. He could also go into a state of self - doubt, wondering what he did wrong.
Natsu could handle the rejection by using it as motivation. He might train harder to prove himself to those who rejected him. For example, if he was rejected from a magic competition, he could practice new spells and techniques to show up the judges next time.
She might become angry at first. Natsu is known for her fiery temper. So, she could lash out at the person who betrayed her, confronting them directly and demanding an explanation.
His impulsiveness might decrease. As an old man, Natsu would have more life experiences. He might think twice before jumping into a fight. For example, he would consider the consequences for his friends and the guild.
Natsu's fire magic could enhance Naruto's fire - based jutsu. For example, if Naruto uses his Fireball Jutsu, Natsu could add his own magical fire power to make it even more powerful. Also, Naruto's shadow clones could be used in interesting ways with Natsu's fighting style. Natsu could use the clones to his advantage, like having them distract an enemy while he charges in with a powerful fire punch.
He may try to understand Erza's reasons. Natsu, despite his impulsive nature, has a big heart. He might respect Erza's decision and step back, trying to keep their friendship intact. However, he could also be a bit persistent in a non - annoying way. He might keep showing his care for her in a more friendly manner, hoping that over time, she might change her mind.