Well, a plot twist could be that Laxus is forced to go on a dangerous mission right before the wedding. This would leave Lucy to deal with all the wedding preparations alone and also worry about his safety. Or perhaps there is a prophecy that says their union will bring either great good or great evil to the world of magic. This would make them second - guess their decision to get married. Also, what if a magical artifact that Lucy discovers has the power to erase Laxus' memories? And she has to decide whether to keep it hidden or tell him, which would be a huge plot twist in their relationship.
One possible plot twist could be that an old enemy resurfaces on their wedding day, threatening to disrupt the ceremony. Maybe a dark guild that Laxus once fought against comes back for revenge. Another twist could be that Lucy discovers a hidden family secret that might change her relationship with Laxus. For example, her family has some sort of connection to Laxus' past that she was unaware of. And a third plot twist could be that a magic spell goes wrong during the wedding, causing everyone to swap personalities for a while, which would lead to some really interesting and comical situations.
A plot twist might be that an unknown relative of Laxus shows up and objects to the marriage. This relative could claim that there is some sort of ancient family curse that will befall them if they get married. Another interesting twist could be that Lucy suddenly loses her magic powers right before the wedding. This would make her question whether she is still worthy of being with Laxus, who is a very powerful mage. And then there could be a situation where the wedding venue is magically transported to a different dimension, and they have to find a way to get back to their world and complete the wedding.
Chapter 227, "There's Something Wrong with This Stepmother", is the clearest early answer because it turns Something Wrong's situation into a concrete plot event. It opens with Your Four Pillars of Destiny is very strong, with wealth, authority,...
Chapter 217, "No Problem, They're All Red Anyway", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event.
Chapter 495, "It's Not Too Much to Ask for Just a Thousand Points, Right?", is the clearest early answer because it turns Miss Su's situation into a concrete plot event.
Chapter 647, "Let's Have a Sword Dance!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
Chapter 83, "Can Actually Carve Flowers?!", is the clearest early answer because it turns Shu Yan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with When they received the task, the three of them were already...
Chapter 438, "The Upper Stream is Mine, The Lower Stream is Yours", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event. The chapter gives readers a real answer by opening on I'm sorry," Shang Shuyue squeezed out three words through clenched...
Chapter 297, "Is She Really His Granddaughter?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
Chapter 118, "Director Sun, When Did You Get a Barrage Above Your Head?", is the clearest early answer because it turns Get's situation into a concrete plot event.
Chapter 115, "Jiang Yan's New Follower—Richard!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, Jiang Yan was woken by a loud crowing noise.. By the end, [Pure bystander, wasn't Jiang Yan known as a notorious playboy?.