The plot might involve Zeref kidnapping Natsu to study his connection to the dragons. Since Natsu has a deep bond with Igneel and other dragon - related magic, Zeref, who has his own complex relationship with dragons and magic, could be intrigued. He might keep Natsu in a hidden lair, and Natsu would have to find a way to escape while also dealing with Zeref's psychological games.
In a 'Zeref kidnaps Natsu' fanfiction, perhaps Zeref kidnaps Natsu because he sees Natsu as a key to his own redemption or destruction. He could be thinking that by having Natsu close, he can either find a way to end his own cursed existence or drag Natsu down with him into the darkness.
Well, one possible plot could be that Zeref kidnaps Natsu in an attempt to use his power for some dark purpose. Maybe he wants to turn Natsu into a weapon against Fairy Tail. Zeref might believe that Natsu's unique dragon - slayer magic can be harnessed in a way that no one else's can.
Well, perhaps Zeref kidnaps Natsu because he sees Natsu as a key to some grand plan. Maybe he wants to use Natsu's power for his own dark purposes. For example, Zeref might think Natsu's unique magic can help him break some sort of ancient curse or gain access to a powerful magic source.
Perhaps Natsu kidnaps Lucy as a prank that goes wrong. He initially thinks it'll be a funny joke, like taking her away during a guild party just for a bit of chaos. But then things get out of hand. Lucy might be really angry at first, but as they are alone together, they start to have some deep conversations and understand each other better.
Gray would probably be one of the first to want to go after Natsu. He and Natsu have a long - standing rivalry but also a deep friendship. He'd be furious and determined to rescue Natsu. Lucy would be very worried too. She might use her celestial spirit magic to try and find Natsu's location. Erza would be very focused and strategic, rallying the rest of Fairy Tail to come up with a rescue plan.
In a Natsu joins Zeref fanfiction, one possibility is that Natsu could initially be conflicted. He has always seen Zeref as an enemy, but perhaps there's a common goal they discover. For example, they might find out about a greater evil that threatens the entire magical world. Natsu's fiery nature could be used in a new way when combined with Zeref's dark magic. This could lead to a very different power dynamic compared to what we see in the original Fairy Tail story.
Well, it depends on the specific fanfiction. Some might have a plot centered around their unique friendship in a different AU (alternate universe), like they team up to fight a common enemy. Others could explore their relationship in a more emotional way, perhaps with Zeref having to deal with her new female form and Natsu being there to support her in ways different from the original story.
If Natsu and Zeref join Fairy Tail in fanfiction, it would change the dynamic of the guild completely. The other members would have mixed reactions. Some might be distrustful of Zeref due to his past actions. However, as time passes, they could see the good in him. Natsu and Zeref together could attract a lot of attention from outside forces. Enemies might target Fairy Tail more because of their combined power. But also, they could inspire the guild to become even stronger, with everyone striving to keep up with their new powerful comrades.
Maybe some unexpected allies could emerge. For instance, other dark mages who oppose Zeref's actions for their own reasons. They might see Natsu as a valuable asset or simply dislike Zeref's overbearing ways. Another potential ally could be a powerful spirit that Natsu has befriended in the past. Spirits often have unique powers that could be useful in the fight against Zeref.
It could be another dark mage who sees the potential in baby Natsu and wants to use him for evil purposes. This mage might view Zeref as an obstacle in getting to Natsu.
Another plot twist could be that the past they remember isn't actually real. It was all an illusion created by a powerful mage. When they find out, they have to figure out who the real enemy is and why they were put in this false past. This would completely shift the focus of the fanfiction from their relationship in the 'past' to finding out the truth behind the illusion.
One idea could be that baby Natsu has some sort of special connection to Zeref that even Zeref doesn't fully understand at first. Maybe Natsu's presence starts to change Zeref's dark thoughts a bit.