In a lot of these fanfictions, Acnologia is a common antagonist. He's such a powerful dragon in the Fairy Tail universe that he makes a great villain. Another one could be Zeref. His dark magic and his complex relationship with the main characters can be explored in Nalu fanfictions related to Dragon Cry.
Often, the main antagonists are characters related to the Dragon Cry plot. For example, the people who are trying to misuse the power of the Dragon Cry itself. They might be a group of dark mages who want to use it for their own evil purposes. Also, there are fanfictions where old enemies of Fairy Tail resurface as antagonists, like the dark guilds that have a vendetta against Natsu, Lucy and the guild.
Well, in Fairy Tail Dragon Cry Nalu fanfiction, themes like friendship are quite prevalent. Since Natsu and Lucy are such good friends in the original, fanfic writers like to expand on that. Family is another theme. They create scenarios where Natsu and Lucy find a new kind of family within the guild. And of course, the theme of facing great evil together. They have to battle powerful foes and that makes for some exciting stories.
Natsu is often a main character. His fiery nature and love for adventure would make him a central figure in any cabin - related story. Lucy might also be featured prominently as she has a close relationship with Natsu and her celestial spirit powers could come in handy in the cabin setting.
Natsu is often a main character in Fairy Tail Woods fanfiction. His adventurous spirit makes him a natural fit for exploring the woods. He might be on a quest to find something powerful or just looking for a new challenge.
Lucy is also a common character. Her celestial spirit magic gives a lot of room for creativity. Fanfiction might show her summoning spirits in new and unexpected ways or forming deeper bonds with them.
The black dragon itself can be a main character. Writers might explore its backstory, its motives, and its personality. It could be a misunderstood creature, or a powerful and menacing force that the Fairy Tail guild has to deal with. In some fanfictions, the black dragon might have a name and a complex history that unfolds as the story progresses.
Sometimes new, original antagonists are created for dark fanfiction. They could be powerful demons or shadowy figures from Vegeta's past on Planet Vegeta. These antagonists might have vendettas against Vegeta and see harming Bulma as a way to get revenge. They can bring in elements of mystery and horror to the story.
It could be a group of students from another academy who are jealous of Fairy Tail Academy's reputation. They constantly try to undermine Natsu and Lucy's efforts and cause trouble during competitions.
It could be a group of rogue mages who have a vendetta against Natsu and Erza. They see the two leaving Fairy Tail as an opportunity to attack them when they are more vulnerable outside the protection of the guild.
Lucy might also be a common character. Her celestial spirit magic could interact in interesting ways with the glittery magic of the Fairy Glitter world in the fanfiction. For example, she could summon spirits to help Natsu in his adventures there.
Obviously, Natsu is one of the main characters. He is often the center of the action in these fanfictions. Then there are the female characters from Fairy Hills, like Lucy. She usually plays an important role, maybe as Natsu's partner or as someone who gives him advice on the mysteries of Fairy Hills.