A powerful dark guild could be the main antagonists. They might be jealous of the combined power of Natsu and Mirajane's family and seek to destroy them to gain more power in the magical world.
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.
For one, it's the characters themselves. Natsu and Mirajane are both popular characters in Fairy Tail. When you put them in a heartbreak scenario, it adds a new layer of depth to their relationship. Fans are interested in seeing how this could potentially change their dynamic. Also, it allows for a lot of creativity in the storytelling. Writers can explore different reasons for the heartbreak, like misunderstandings, external forces, or personal sacrifices, which all make for an interesting read.
I'm not sure of specific ones off - hand. But you can try searching on fanfiction - dedicated websites like Archive of Our Own or FanFiction.net. They usually have a wide range of fanfictions including 'natsu x mirajane' ones.
One of the main things that makes 'Fairy Tail Natsu x Mirajane fanfiction' appealing is the opportunity to create a unique love story within the magical world of Fairy Tail. Fans can imagine how their magic would interact in a romantic context. For example, Natsu's fire magic and Mirajane's take - over magic could be used in creative ways to save each other or have special moments together. Also, seeing these two characters, who are both important to the guild, develop a deep emotional connection is very satisfying for readers.
One great 'Fairy Tail Natsu x Mirajane fanfiction' is 'A New Bond'. It beautifully explores their relationship in a post - Tartaros arc setting, showing how they support each other through the rebuilding of the guild. Another interesting one is 'The Unseen Connection' which delves into their shared past traumas and how that brings them closer.
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.
Rather than specifically seeking 'natsu x mirajane lemon fanfiction', which is likely to be hard to find in a proper and legal way, you could join fan communities dedicated to the series. In these communities, you can ask for recommendations on good 'natsu x mirajane' fanfiction that focuses on their relationship, friendship, or any other positive aspects. This way, you can enjoy stories about them without having to deal with any potentially inappropriate content.
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 popular theme could be their shared adventures. Since they are both part of the same world in the series, stories could focus on them going on quests together. Another theme might be the development of their friendship into something more. For example, how they gradually realize their feelings for each other through different events and interactions.