A plot could be that during a difficult mission, Mirajane discovers she's pregnant. Natsu then has to balance protecting her and completing the mission. This would add a layer of tension and emotion to the story.
One idea could be that they discover the pregnancy during a big battle. The news gives them extra motivation to protect their world and each other. They start to prepare for the baby's arrival while still dealing with the threats around them.
Well, there could be such a fanfiction out there. However, it really depends on the creativity and interests of the fanfiction writers in the relevant fandom. Some might find this concept interesting to write about, while others may not. It also might not be very common as it involves a rather specific and perhaps controversial plot point in the context of the characters.
One plot idea could be that Natsu accidentally discovers Mirajane's secret hobby, like painting. As he tries to help her show her work, they grow closer. Another could be a mission where they get separated from the rest of the guild and have to rely on each other for survival, leading to a realization of their feelings. Or perhaps a magic - induced amnesia situation where Natsu forgets everyone but Mirajane is determined to make him remember their relationship.
I'm not entirely sure if there is a specific 'natsu and mirajane pregnant fanfic'. Fanfic availability can vary greatly. Some fanfic authors might have explored this idea, while others may not. You could also try reaching out to fanfic communities on social media platforms and ask if anyone knows of such a fanfic.
One idea could be that Natsu and Mirajane start a family in a peaceful post - guild - war era. Their kids inherit unique magical abilities from both of them and they have to train and guide their children through the magical world's challenges.
One plot idea could be that Natsu has to deal with the physical and emotional changes of pregnancy while still trying to be part of his usual adventures. Maybe his friends are super protective of him now.
Well, a great plot could involve them facing some challenges during the pregnancy. For instance, Mirajane might have some difficulties with her magic due to the pregnancy hormones. And Laxus would be super protective, which could lead to some interesting conflicts within the guild when he overreacts to perceived threats. Also, they could discover some old family traditions related to pregnancy and childbirth that they decide to incorporate.
One idea could be focusing on how her pregnancy affects her relationships within the guild. Maybe she becomes more protective and nurturing towards the younger members. Another could be about the challenges she faces in balancing her pregnancy with her duties as a mage. For example, she might have to take a break from dangerous missions but still find ways to contribute to the guild's well - being.
Some alternatives could involve fanfictions centered around their individual character development. For example, a fanfiction about Natsu learning a new powerful spell or Mirajane exploring a new side of her magic. There could also be stories that place them in different settings, like a modern - day AU (alternate universe) where they are just regular people dealing with normal life problems while still maintaining their core character traits.
Erza would probably be very protective. She would make sure Mirajane is taking proper care of herself and would offer practical advice on things like diet and exercise during pregnancy. Gray might be a bit awkward at first but would warm up and offer to help with any heavy lifting or tasks around their home.