Naruto's friends from Konoha would likely be very supportive. Kakashi might offer some wise words about the responsibilities of parenthood. Sakura would probably be there to help with any medical concerns Mirajane might have during the pregnancy. As for Mirajane's friends from Fairy Tail, they would be excited. Erza would be making sure Mirajane is well - protected and would probably train Naruto a bit to make sure he can defend his family. Gray might make some jokes to lighten the mood but also be ready to help out.
One possible plot twist could be that there is some sort of magic or jutsu involved that endangers the pregnancy. For example, a dark magic user might target Mirajane because of her unique powers and the fact that she is pregnant, seeing the unborn child as a potential threat in the future. Another twist could be that Naruto discovers a new form of chakra that can only be accessed through the combined efforts of him and Mirajane during her pregnancy, which then becomes crucial for saving their world from a great evil.
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 possible fanfiction could be about Naruto visiting the Fairy Tail world and teaming up with Erza and Mirajane. They could go on a quest together to defeat a powerful dark wizard. Naruto's unique ninja skills would blend well with Erza's swordsmanship and Mirajane's takeover magic.
They could start by meeting in a strange, in - between world. Naruto is intrigued by Erza's armor and Mirajane's different forms. They soon realize they have a common goal, which is to find a way back to their own worlds. As they journey, they share stories of their past adventures, with Naruto talking about his battles in the Hidden Leaf Village and Erza and Mirajane sharing tales from Fairy Tail.
One possible fanfiction could be about Naruto ending up in the Fairy Tail world and teaming up with Erza and Mirajane. They could go on adventures together, with Naruto's unique ninja skills complementing the magic of Erza and Mirajane. For example, Naruto's shadow clone jutsu could be used in creative ways during battles against powerful foes in the Fairy Tail universe.
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 a crossover where Naruto, Erza, and Mirajane are brought together by a powerful magic spell that goes wrong. They have to work together to reverse the spell's effects while also dealing with the differences in their worlds. Naruto's ninja skills, Erza's swordsmanship and armor changing abilities, and Mirajane's demon form and magic could create really interesting interactions and battle scenes.
In a more complex fanfiction, Naruto, Erza, and Mirajane could be brought together by a prophecy. The prophecy states that only by combining the power of a ninja from Naruto's world and the mages from Fairy Tail can a great evil be defeated. At first, they struggle to understand each other's ways, but as they train together, they start to blend their skills. Naruto might teach them some stealth techniques, while Erza and Mirajane show him how to better utilize chakra - like energy in their magic. This would lead to some really epic battles against the evil force as they learn to fight as a team.
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.