The plot could involve the entire Fairy Tail guild being involved in their pregnancy journey. For example, Levy could be helping Mirajane with researching about any magical implications of a half - dragon (assuming Natsu's dragon side has an impact) and half - demon (Mirajane's side) baby. Erza might be training Natsu to be more responsible and gentle. And as the due date approaches, the guild could be preparing a grand celebration for the new arrival while also ensuring the safety of Mirajane and the unborn child from any potential threats.
One possible plot could be that Natsu and Mirajane join Fairy Tail at different times. Natsu might bring his usual enthusiasm and powerful fire - based magic. He could quickly get into all sorts of adventures, making friends and enemies alike. Mirajane, on the other hand, could use her Take - Over magic to be a great asset in battles. Their joining could lead to new team - ups within the guild, like Natsu and Mirajane going on a mission together to stop a dark wizard who is causing chaos in a nearby town. They'd face challenges, but their combined skills would help them overcome the odds.
One possible plot idea could be that they face challenges in their pregnancy journey. Maybe Lisanna has a difficult time physically due to the magical nature of their world affecting her body. Natsu, on the other hand, tries his best to support her but also has to deal with his own guild duties. Another idea is that their unborn child shows signs of having extremely powerful magic even before birth, which causes some concern among the guild members as they worry about potential threats that might be attracted to such power.
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.
In fanfiction, Mirajane and Natsu could have a very interesting mentor - mentee relationship. Mirajane, being more experienced, could teach Natsu some new magic techniques or how to better control his power. This would add a new dynamic to their interactions within the Fairy Tail guild.
Perhaps they leave because of a disagreement within the guild. Maybe there's a new policy or a decision made by the guild master that they don't agree with. For example, if the guild decides to get involved in a war that they think is unjust, they might choose to leave rather than participate against their beliefs.
Their relationship would likely become more intimate and deeper. With a baby on the way, they'd have to rely on each other more. Natsu would probably become more gentle and caring towards Mirajane, and Mirajane would feel more protected by Natsu.
One idea could be that in a time - skip, Natsu and Erza have been secretly in love for a while. Erza discovers she's pregnant. They're initially scared but then excited. Natsu vows to be the best father. Their guild mates are shocked at first but then super supportive, helping them prepare for the baby with lots of magical items and advice.
One way is through the child's magic. If the child has a magic related to entropy, it could be shown as a force that is hard to predict. For example, the child might not be able to fully control when they use their entropy magic, and it could randomly affect things around them, like changing the temperature or the state of objects in the guild hall.
One characteristic is the exploration of their personalities. Natsu's brash and hot - headed nature contrasts with Mirajane's more composed and sometimes mysterious side. In fanfiction, this contrast often leads to interesting interactions. For example, Natsu might be the one to break through Mirajane's reserved exterior with his straightforwardness.
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.