The main characters are clearly Natsu and Lucy. Natsu, with his fiery and impulsive nature, is likely to be there to encourage and sometimes push Lucy in her journey to get stronger. His dragon slayer magic might also play a role in training her or protecting her during dangerous situations.
One possible plot could be that Lucy trains intensively with Natsu by her side for support. They might go on special quests to find magical items or training methods that can enhance Lucy's power. For example, they could search for an ancient spell book that has the power to unlock new celestial spirit powers for Lucy.
Makarov could be her mentor. He is the master of the Fairy Tail guild and has a vast amount of magical knowledge and experience. He could guide Lucy in exploring deeper aspects of her celestial spirit magic, teaching her how to better communicate with her spirits on a more profound level, which would in turn make her stronger when summoning and using their powers.
Well, first of all, Lucy can try to have an open and honest conversation with Lisanna. By expressing her feelings and asking Lisanna why she is acting this way, she might be able to resolve the conflict. If that doesn't work, she can choose to rise above it. She can pour her energy into her magic training, become stronger, and show that she is not going to be brought down by Lisanna's actions. She can also look for new friendships outside of the group that may give her more positive energy and support. Another option is for Lucy to take a break from the situation. She can go on a solo adventure, clear her head, and come back with a new perspective on how to deal with Lisanna.
One common theme could be family. Since it involves 'baby Lucy', it might explore how Natsu and Lucy build a family together. Another theme could be adventure in a new way, like how they protect their baby during their adventures.
It's highly likely that there exists a fanfic with a Nalu and Lucy pregnancy plot. Given the large number of fanfic writers and the popularity of the Nalu ship, many fans like to create alternate storylines for the characters. Some might be interested in exploring the idea of Lucy becoming pregnant, perhaps dealing with how Natsu and Lucy would handle the situation, their reactions, and how it would affect their relationship and the world of Fairy Tail around them.
One common theme could be jealousy. Maybe Lisanna is jealous of Lucy's relationship with Natsu and tries to come between them in the fanfiction. Another theme might be misunderstanding. Lisanna could misunderstand Lucy's intentions towards Natsu and act in a way that hurts Lucy. It could also be about competition for Natsu's affection, with Lisanna seeing Lucy as a rival and using underhanded tactics to make Lucy look bad in Natsu's eyes.
One interesting Nalu dark fanfiction could be where Natsu is under a dark spell and Lucy has to find a way to break it. In this story, Natsu might be forced to fight against his friends, and Lucy's love for him is put to the ultimate test as she battles not only the external threats but also Natsu's inner darkness.
Love is a very common theme. Writers often explore the romantic relationship between Natsu and Lucy, showing how their feelings develop over time. Adventure is also common. They go on quests together, fight bad guys, and discover new places.
Lucy could face isolation. People in the fanfiction world might avoid her, and she could be left all alone. This could also lead to her losing confidence in herself as she is constantly being blamed for something she didn't do.