It could be Mystogan. He has that mysterious air about him. Maybe he and Mirajane had a secret encounter during one of his rare appearances. Or perhaps it's someone completely new that the writer creates just for this fanfiction. A wandering mage who passed through and left a lasting impression on Mirajane.
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.
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.
Another option is to check out some Fairy Tail - focused fan communities on social media platforms such as Reddit or Tumblr. Sometimes, fans share links to interesting fanfictions in these communities. However, make sure to follow the community rules when looking for such content.
Their relationship could become more tender. Laxus, who is often seen as tough, might become more protective of Mirajane. For example, he would not let her take on dangerous missions anymore.
Laxus might become more tender and less brash. His focus would shift from just being a strong mage in the guild to being a father. Mirajane, on the other hand, would likely become even more nurturing. She already has a motherly side, but this would be amplified.
The fanfiction might also take a more dramatic turn. For instance, there could be a threat to Mirajane or her unborn child from an enemy of the Fairy Tail guild. This would lead to the guild members rallying around her to protect her and the baby, and Mirajane might have to find ways to use her powers even in her pregnant state to contribute to the defense.
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.
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.
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.