Maybe Naruto, being a devil, has some special powers that are needed to save the Fairy Tail guild from a great threat. Erza is initially skeptical of him, but as they work together, they develop a bond. Along the way, there could be flashbacks to Naruto's past in his own world and how he became a devil, which would add depth to the story.
One possible plotline could be that Erza sacrifices herself to save her guildmates from a powerful dark magic attack. She stands in the way, knowing it will cost her life, but she does it to protect those she loves.
In some fanfictions, the plot might revolve around a magical curse that can only be broken by a true love's kiss between Erza and Jellal. They have to go through various trials and tribulations together, and as they get closer to breaking the curse, their feelings grow until they finally share that kiss. This type of plotline adds an element of adventure and mystery to their relationship.
I'm not sure specifically which ones are great for this particular crossover. You could try searching on fanfiction websites like Archive of Our Own or FanFiction.net. Just use the relevant tags like 'Fairy Tail', 'Naruto', 'Devil', and 'Erza' to narrow down the search.
One popular plotline is the redemption arc of Jellal. Fans often write about how he tries to make up for his past wrongs towards Erza. For example, he might protect her from new threats in the Fairy Tail world. Another plotline is their relationship development. Writers may show how they gradually overcome their past traumas and start to trust and love each other again. Sometimes, they are put in a situation where they have to team up against a powerful enemy, which further deepens their bond.
Erza might be shocked. After all, Naruto is an important part of her life in the fanfiction. She could initially try to convince him to stay, using her own experiences in Fairy Tail to persuade him.
One possible plotline could be Naruto joining Fairy Tail and having to prove himself to the other members. He might start from the bottom and work his way up, showing his skills in battles against other guilds. Another plotline could be that Naruto discovers a connection between his ninja world and the magic of Fairy Tail, and he tries to figure out how to use this connection to his advantage.
One possible plotline could be Naruto getting transported to the Fairy Tail world and having his powers suppressed. He then has to learn Fairy Tail magic from scratch. Another could be that Naruto is cursed in the Fairy Tail world which makes him weak, and he has to find a way to break the curse while making friends in the guild.
One possible plotline could be the redemption of Jellal. After all his past wrongdoings, he tries to make amends and prove himself worthy of Erza. Their kiss could be the moment when Erza finally fully accepts his transformation and they start a new chapter together.
A possible plotline could be that Erza is put in a situation where she has to endure a form of 'torture' to protect her friends. For instance, an enemy might threaten to harm her friends if she doesn't submit to a series of difficult and painful magical experiments. Through this, we can see her selfless nature as she endures the pain for the sake of those she cares about. Also, she could be tortured by a false accusation within Fairy Tail itself, which causes her emotional distress, and she has to clear her name while still maintaining her loyalty to the guild.
A possible plotline is a love triangle. Maybe another character has feelings for Evergreen and Elfman has to compete for her affection. This could create a lot of drama and tension in the fanfiction. For instance, the other character could be constantly trying to undermine Elfman in Evergreen's eyes.