Perhaps a long - lost relative of Lucy shows up and tries to stop the proposal. They could claim that Natsu isn't good enough for her or that there's a family curse that would befall Lucy if she marries a dragon slayer. This would create a conflict between Lucy's love for Natsu and her family loyalty.
One possible plot twist could be that Lucy initially refuses Natsu's proposal because she fears it will change their dynamic in the guild. She might think that getting married will make them less focused on their adventures and more on domestic life.
Another plot twist could be that a dark magic spell is cast on Natsu right after the proposal. This spell makes him forget his love for Lucy, and she has to go on a quest to break the spell and make him remember his proposal. It would add a lot of drama and action to the fanfiction.
The guild would likely be ecstatic. Gray would probably make some snarky comment about Natsu finally growing up, but deep down he'd be happy for his friend. Erza would be all for it and might start planning the wedding immediately, giving orders to everyone in the guild to help out.
Well, one possibility could be that it's a really innocent situation. Maybe they got into some kind of magical mishap that made them dirty and the only available place to clean up was a shared shower area. But they're both really embarrassed and trying to be respectful of each other's space while also dealing with the comical aspect of the situation.
One possible plot could be that Natsu and Lucy become the top students in Fairy Tail Academy. They might use their unique skills and teamwork to overcome various challenges like difficult exams set by strict teachers. For example, Natsu could use his fire magic in a practical magic test, while Lucy summons her celestial spirits to solve problems in a more strategic way.
Well, it could start with Natsu barging into the bathroom thinking it's unoccupied, only to find Lucy already in the bath. There could be a lot of blushing and yelling from Lucy. Then, they might have to figure out how to get Natsu out without alerting the whole guild. Or perhaps, it could be a magical accident that causes them to end up in the same bath, and they have to work together to reverse the spell while dealing with their embarrassment.
One possibility could be that it's a comical situation. Maybe they got into some kind of magical mishap that left them dirty and the only available place to clean up was a small bathroom where they had to share the shower in a very awkward way. Natsu, being his clueless self about social norms, doesn't really understand the awkwardness at first while Lucy is blushing like crazy.
One possibility could be that they discover they're having a baby unexpectedly. Natsu might be initially confused but then super excited. Lucy would probably be a bit more practical, thinking about how to raise the baby in the Fairy Tail guild environment. The guild members would be overjoyed and offer all kinds of help, like Happy bringing little gifts for the baby to be.
In a fanfiction, there could be a situation where Lucy is forced to side with a group that Natsu is against. This would make Natsu see her as an enemy. Or perhaps there is a prophecy that Lucy is involved in that Natsu misinterprets as something bad. He might think she is a threat to the guild or his friends. Also, if there is a love triangle situation where Lucy chooses someone else over Natsu in a way that really hurts his feelings, that could be a reason for him to start hating her.
One interesting plot twist could be that Lucy discovers a hidden power within herself that is linked to End. Maybe she can control a part of End's power and use it to save the Fairy Tail guild from a great danger. For example, she might be able to use it to stop an ancient evil that is more powerful than any they've faced before.
One possible plot twist could be that Natsu initially doesn't believe it's his child. Maybe he has some self - doubts due to his past or his own insecurities. Another twist could be that there are external forces trying to harm Lucy because of her pregnancy, like dark guilds seeing it as a way to get to Natsu. And perhaps the pregnancy could also bring out a new side of Natsu, a more protective and responsible side that no one has seen before.
Well, if Natsu asks Lucy out in fanfiction, it could change the whole dynamic of the Fairy Tail group. Their friends would probably be super supportive, especially Happy. They could start exploring new places not just for quests but also for romantic getaways. And Lucy might write a new story about their relationship in her diary.