There could be a fanfiction where Natsu and Lucy discover a lemon - themed spell. In this story, they have to figure out how to control the spell's power and in the process, their relationship deepens. For example, the spell might cause some humorous or unexpected situations that they have to work through.
You can start by looking on fanfiction websites such as Archive of Our Own or FanFiction.net. Use the search bar and type in 'natsu and lucy lemons' to see what comes up. Check the ratings and reviews of the stories to find the good ones.
They can develop their relationship by having small, everyday moments. For example, sharing breakfast together as a family, where they talk about their plans for the day. Natsu could show his love for Lucy by being more protective of her and their family in dangerous situations. Also, they could have inside jokes that only their family understands, which would strengthen their bond.
One character that would be interesting to include is Mirajane. She has a motherly and observant nature. She could be the one to nudge Natsu and Lucy in the right direction when it comes to their relationship. For instance, she might arrange for them to go on a mission together that she knows will bring them closer. Laxus could also be in the fanfiction. His powerful presence can create a sense of danger or protection. If there's an outside threat to the guild while Natsu and Lucy are exploring their relationship, Laxus could be at the forefront of defending the guild, which would impact Natsu and Lucy's situation. And don't forget about Cana. She could be the one to create some light - hearted chaos, like daring Natsu and Lucy to do something that ends up bringing them closer, such as a bet that leads to a shared adventure.
As they travel, their bond deepens. They start to rely on each other more. Natsu becomes more protective of Lucy, and Lucy begins to understand Natsu's impulsive nature better.
Obviously, Lucy Lemons is a main character. But often, there can be her best friend, like Lily, who is always by her side in the fanfic. There might also be a mentor figure, perhaps an old lady named Mrs. Green who gives her advice.
A Natsu x Lucy pregnant fanfic could be full of heartwarming moments. Think about Natsu being overprotective of Lucy during her pregnancy. He'd probably be a bit clumsy at first, but his love for her would shine through. And Lucy would be the one trying to keep things in order while also dealing with her hormonal changes. It could also add a new dynamic to the Fairy Tail group as a whole.