Well, perhaps they are transported to a parallel world where they are already the Dragon King and Queen. But this world is in chaos due to an evil dragon's reign. Natsu and Lucy have to figure out how to use their newfound status and powers to defeat the evil dragon and restore peace. They might also meet alternate versions of their friends who have different roles in this world.
A possible plot could be that Natsu and Lucy are chosen by the dragon spirits to be the King and Queen. As they accept this role, they start to notice changes in themselves. Natsu's fire magic becomes more powerful and dragon - like, and Lucy is able to summon stronger celestial spirits. They have to learn to rule over the dragons, deal with dragon politics, and also protect their own world from threats that come with their new position.
Chapter 684, "It doesn't matter if it gets broken, I have the money to fix it.", is the clearest early answer because it turns Luo Xingmu's situation into a concrete plot event. It opens with But Jiang Li had already evolved; she was now a film producer who...
Chapter 648, "Even Wild Boars Make Way!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with Jiang Li also casually picked up a tree branch and then directly... and closes on This is just a program performance!] Jiang Li and Brother Pei....
Chapter 316, "Zhao Lang Gets Jealous", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event.
Chapter 854, "These Brothers Are Quite Lacking in Love", is the clearest early answer because it turns Ji Yanbai's situation into a concrete plot event.
Chapter 513, "A Public Stage", is the clearest early answer because it turns Tai Fan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Everyone was sharing their experiences in a few words, the banter...
Chapter 315, "It seems a bit difficult to get a perfect score", is the clearest early answer because it turns Madam Jiang's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Li, however, looked at the delicate draft and said, "Mom, why...
Chapter 603, "Met Uncle Beggar", is the clearest early answer because it turns Jiang Mingchuan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Our family's ships have flipped, but it should be just two of them."...
Chapter 685, "Defense Value Skyrockets!", is the clearest early answer because it turns Luo Xingmu's situation into a concrete plot event. The chapter turns that into something concrete by moving from The scene went live on Weibo right then. to How could he not be furious?.
Chapter 422, "The Watch Was a Gift from Him", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with Seeing that no voice came from the watch to stop them, the captain of...
Chapter 641, "Congratulations to Jiang Li for Taking the Top Spot!", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event.