The guild master, Makarov, would be deeply concerned. He might use his own magic to try and stabilize Natsu's condition. Some of the more impulsive members like Natsu's friends might want to take immediate revenge on whoever or whatever they think caused the heart attack. Meanwhile, characters from other guilds who are friends with Natsu might come to offer their assistance, bringing with them their own unique magics and resources to help save Natsu.
Perhaps Natsu gets a heart attack because of an emotional shock. For example, if he discovers something truly devastating about his past, like a dark secret related to Igneel. The sudden rush of intense emotions could be too much for his heart to handle. Also, it could be a result of a magical experiment gone wrong. He might be trying out a new type of magic that has unforeseen consequences on his physical well - being, especially his heart.
He could marry an original character created specifically for the fanfiction. This character could be a powerful mage from a far - off place who crosses paths with Natsu during one of his adventures.
One of the senior and more experienced members of the guild. They have seen a lot and know how things should be done in the guild. If Natsu acts in a way that goes against their long - established ways, they would not hesitate to berate him.
It could be Gray. Although they are friends most of the time, in a fanfiction, there could be a plot twist where Gray's envy of Natsu's abilities gets the better of him, and he betrays Natsu. Maybe Gray thinks that by getting rid of Natsu, he can be the top mage in their guild.
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.
There could be such a fanfiction. Fanfiction writers often come up with all kinds of creative and sometimes out - of - the - box ideas. However, I haven't personally come across one where Natsu gets Erza pregnant. It might be a rather niche concept within the Fairy Tail fanfiction community, as most fanfictions tend to focus on different aspects of the characters' relationships, like friendship, adventure - related partnerships, or other forms of romantic development.