One way to write it could be through a portal - based story. A portal opens between the Naruto and Fairy Tail universes. Naruto and Erza meet, and due to some strange energy in the portal area, a bond forms that results in pregnancy. This would be a very basic and direct way to start the story.
Another approach could be a dream - like scenario. Naruto has a strange dream where he meets Erza, and in this dream - world, they end up in a situation where there's a pregnancy. When he wakes up, he discovers that the dream has some sort of real - world connection, and he has to figure out how to deal with it. This would add an element of mystery to the fanfiction.
There are no official stories of Naruto and Erza having a relationship as they are from different anime universes. However, in fanfiction, it could be an interesting concept. If we were to imagine, perhaps a crossover magic or some sort of interdimensional event could bring them together, leading to this unexpected pregnancy situation. But this is all in the realm of creative fanfiction writing.
Naruto might be curious about Erza's armors and Mirajane's demon form. He would probably ask a lot of questions. Erza would be impressed by Naruto's tenacity and his ability to never give up. Mirajane could bond with Naruto over their shared sense of protecting their friends. They could all train together, with Naruto showing his ninja training methods, and Erza and Mirajane teaching magic - based combat.
In this fanfiction, perhaps Erza and Gray's pregnancy could be the result of a magical accident. They would then need to figure out how to handle this unexpected situation. Their relationship might grow stronger as they face this together, sharing their fears and hopes for the future with each other and their close friends.
Well, in fanfiction, the author might create this situation to explore new storylines. Maybe the author wants to show Naruto's vulnerability. Erza could be made to betray Naruto due to some misinformation or a dark influence. For example, if there's an evil force that manipulates Erza into believing Naruto is a threat to her world or friends.
Naruto might be really excited to meet Erza and show off his ninja skills. Kushina could be a bit more cautious, but she'd also be curious. Erza would probably be impressed by Naruto's determination, similar to her own. They could start by sharing stories about their respective worlds, like Naruto talking about his training with Jiraiya and Erza about her time in the Fairy Tail guild. Kushina could chime in with her own experiences in the Hidden Leaf village.
Perhaps there is a prophecy that only Naruto and Erza together can fulfill. They start off as strangers but as they journey to fulfill the prophecy, they discover that they have a lot in common. They both have a strong sense of justice and a will to protect their friends. As they face numerous enemies and obstacles, their friendship turns into a deep respect for each other's abilities, and they become a formidable team.
One possible reason could be a misunderstanding. Erza might misunderstand Naruto's intentions. For instance, if Naruto is on a mission that involves some secretive actions, Erza could misinterpret them as something malicious and decide to betray him.
Erza Scarlet's tattoos can be incorporated into a Naruto fanfiction in diverse ways. Maybe the tattoos are a result of an experiment by Orochimaru - like figure in the Naruto world, trying to combine different forms of power. They could also be a form of spiritual connection to a new type of summon in Naruto, say a powerful beast that only responds to those with such tattoos. Additionally, her tattoos could be a map or a code in the Naruto world that leads to a great treasure or a secret place filled with powerful ninja techniques.
Well, in this fanfiction concept, it could work in terms of Naruto's protective nature. Naruto is known for his loyalty and his will to protect those he cares about. If he were Erza's older brother, he would surely use his skills and abilities to keep her safe. He could teach her some of his ninja techniques, which would be really cool to see combined with Erza's own combat skills. Also, Naruto's positive attitude could influence Erza in a different way, making her more open and perhaps less brooding at times.
It could start with Naruto being found in the Fairy Tail world as a baby, with Erza taking him in as her own. She might notice something special about him right away, like his strong will that reminds her of herself. Then as he grows up, he starts to show signs of both ninja and magic abilities, which puzzles everyone in the guild but also makes them excited to see what he'll become.