Perhaps they time travel to a time when a powerful dark magic was first discovered. Natsu, with his dragon - slayer magic, and Erza, with her various armors and combat skills, have to prevent the dark magic from falling into the wrong hands. They might face ancient mages and creatures that are more powerful than what they're used to. It would be a great adventure full of challenges and they would learn a lot about the origin of different magics.
One possible plot could be that Natsu and Erza are sent back in time to a crucial moment in the history of their guild. Maybe they go back to the time when the guild was first founded. They could end up having to protect the founders from a threat that was long forgotten in their own time. This would give them a chance to see how the guild started and also change some things for the better in the present.
The plot could also involve time travel to a future version of their world that has been destroyed by some catastrophe. Natsu and Erza then have to find out what caused it and travel back in time to stop it from ever happening. Along the way, they would see how their actions in the present could lead to such a disastrous future and make different choices to avoid it. They might meet future versions of their friends and enemies which would add more depth to the story.
One possible plot could be that they discover the pregnancy and then face challenges together. Maybe they have to deal with the reactions of their friends in the guild, some being happy and supportive while others might be worried about how it will affect their adventuring lives.
Perhaps in the fanfiction, Natsu meets Erza in a completely new setting, like a parallel universe where the Fairy Tail guild doesn't exist yet. They start off as strangers but quickly realize they have similar goals. Erza might initially be a bit distrustful of Natsu's wild ways, but as they face challenges together, such as a group of bandits attacking a village, they learn to trust and rely on each other.
One possible plot is that a time - travel or alternate - reality situation occurs where Natsu and Erza are already married and have a baby. The story could show how they adjust to this new family life. Natsu might be the fun - loving dad who plays with the baby using his fire powers in a safe way, while Erza is the strict but loving mom who makes sure the baby is well - behaved and learns proper values. They would also have to deal with the reactions of their friends in the guild, some of whom might be shocked or overly excited about the new addition to their family.
Well, perhaps Erza was tricked into believing something false about Natsu. Maybe she was misled by some enemy who made it seem like Natsu was going to endanger the guild. So, in her sense of duty to protect the guild, she took actions that Natsu perceived as betrayal.
It could be about a misunderstanding. Maybe Natsu is under a lot of stress or has been wrongly accused, and Erza's actions or words accidentally trigger his outburst.
Well, it could be about a powerful encounter. Maybe Erza initially mistakes End Natsu for the regular Natsu and there's some confusion. Then they might have a fight as Erza is always ready to combat threats. But as they engage, they could discover things about each other that change their views.
One possible plot could be that Natsu starts having dreams about Erza. These dreams make him realize his feelings for her. He then tries to get closer to her by going on more missions together. Along the way, he discovers little things about Erza that he adores, like her love for strawberry cake.
Well, one possible plot could be a love triangle situation. Erza and Mirajane both have feelings for Natsu, but Natsu is oblivious to their affections. They might compete in a friendly way at first, like trying to be the one to give Natsu the best meal or help him with his training. This could lead to some humorous moments as Natsu just doesn't pick up on their real intentions.
One possible plot could be that Natsu and Mira accidentally stumble upon a magical artifact that sends them back in time. They might end up in a time when Fairy Tail was just being founded. There, they could meet the younger versions of their guildmates and have to work together to make sure the guild's history unfolds as it should, while also trying to find a way back to their own time.
One great 'Natsu x Erza time travel fanfiction' is 'Time's Twist'. In it, Natsu and Erza are sent back in time due to a magical mishap. The story beautifully explores their relationship in a different era. They have to deal with the past versions of themselves and their guild mates while also trying to find a way back to their own time. It has some really exciting action scenes and heart - warming moments between Natsu and Erza.