Well, one possibility is that Naruto uses his ninja skills like shadow clones to distract the enemies that are after Lucy. He could then use his Rasengan to break through any barriers between Lucy and safety.
Maybe Lucy finds a long - lost spell book that belonged to a powerful Celestial Mage in the past. The spells in this book are the key to saving everyone. She spends time deciphering the ancient spells and finally manages to use one that can nullify the threat facing Fairy Tail. It could be a spell that can erase the existence of a powerful dark magic or something like that.
In a battle against a group of powerful enemies, Natsu overexerts himself and is on the verge of being defeated. Lucy steps in. She first summons all of her available Celestial Spirits to create a powerful defensive formation around Natsu. Then, she combines her own magical attacks with those of her spirits to drive the enemies away. After that, she uses her healing magic (if she has developed such in the fanfiction) to nurse Natsu back to health.
One common theme is friendship. Many fanfics explore how Snow and Lucy become friends and how that friendship evolves over time. They might start as strangers or acquaintances and then develop a deep bond.
Power and morality. Snow's journey towards power and how it clashes with his relationship with Lucy Gray. Fanfictions might show different perspectives on whether his actions are justifiable or not in the context of his feelings for her.
The theme of survival is also quite prevalent. They were both in a world where survival was not guaranteed. In fanfiction, this can be seen in how they each tried to use the other to survive, whether it was Lucy Gray using her charm and wits or Coriolanus using his position and power. It adds an interesting layer to their relationship and the choices they made.