Swan Lake AU
This is based off of various versions of Swan Lake that I've seen.
Gray and Juvia are the heirs to two separate kingdoms and their parents are have the children be play mates with each other in hopes that they'll fall in love in adulthood. During their childhood Juvia develops a crush on Gray and at first he's very fond of her. He even gives her a locket as a present. But Gray's other friend Natsu makes fun of him for liking a girl. Feeling embarrassed, he starts to acts like he can't stand her and avoids her however he can. However her feelings for him remain the same. Then one day when they're preteens Juvia's family is attacked and she disappears. Both her kingdom and Gray's search for her but she's never found, leaving the young prince heartbroken.
Five years go by, now a young man Gray's mother Ur is trying to convince her son to marry but he's not interested and would rather spend his time hunting and practicing archery with Natsu. One day while out hunting he spots a swan and nearly shoots it but a feeling in his heart makes him change his mind. When the swan flies away it drops something and Gray recognizes it as the locket he had given Juvia many years ago. It then dawns on him that he has seen that swan before and he learns from the royal staff that swan has been seen flying the castle nearly every week for the past five years. Gray is soon convinced that the swan might have a connection to where Juvia is.
Little does he realize that the swan is in fact Juvia. Five years prior she had been kidnapped by the evil sorcerer Brain who plans to marry her once she reaches the proper age and to ensure that no other man falls in love with her he curses her to become a swan during the day and only turn back into a human at night. She is kept in an Enchanted forest and Brian warns her that if she ever leaves the forest at night he will kill her family and destroy her kingdom. Juvia finds refuge with Erza the fairy queen and the true ruler of the Enchanted forest who was overthrown by Brain and had her subjects changed into exceeds. Brain's goal is to rule both the Enchanted forest by stealing Erza's crystal the source of power and a human kingdom by marrying Juvia.
Despite being separated from her family and trapped under a curse Juvia finds happiness by observing Gray in her swan form which over the years makes her love for him grow stronger. One day as she's flying back to the forest he follows her and sees her transform into a beautiful young woman at nightfall. They are both thrilled to see each other again But Juvia tries to get him to leave because she fears that Brain will harm him. He refuses and promises to find someway to save her. He visits her every night and falls deeply in love with her. Erza notices this and learns from an ancient book that if Gray vows to love Juvia forever and proves his love then all of Brain's spells will be broken. But if he swears his love to another then Juvia will die and the magic in Erza's crystal will be lost.
With this information Gray invites Juvia to come to a ball his mother is holding and is expecting him to choose a bride but tells her that he plans to make his vow to her there in front of everyone. Juvia is unsure at first but he convinces her to come. Unfortunately their conversation is overheard by Brain's daughter Briar who reports it to her father. He then confronts Juvia and demands that she forget Gray and marry him instead. She refuses, saying "I'd rather die." Infuriated he decides to grant her request and finally defeat Erza by tricking Gray into swearing his love to another woman: Briar.
Juvia is locked in Brain's castle in swan form while he and Briar go to the ball. Erza and the exceeds help Juvia escape and she flies to the castle. At the ball Brain uses magic to disguise Briar as Juvia and Gray is so happy to see her that he fails to notice that it is not yet night time so Juvia should not be human right now. The real Juvia arrives at the castle but can't get in. She tries to get Gray's attention but Briar makes sure to keep his attention on her. Gray announces that he has decided to marry the woman he is dancing with and vows to love her forever. Briar then reveals her true form to Gray and Brain appears to tell him that Juvia will now die because of him. Juvia flies away, Gray follows her, when she returns to the forest she turns back into a human and collapses. Furious, Erza attempts to battle Brain but is easily defeated due to losing her powers. She along with the exceeds are then turned into statues as Brain proceeds to takeover the forest.
Gray finally arrives and finds Juvia barely alive. She tells Gray that she doesn't blame him for her death and that she still loves him and always will. She dies. Gray cries and Brain taunts him about "Killing the woman he supposedly loved." Enraged and heartbroken, Gray demands that Brain revive her. Wanting to hurt Gray further for his own sick amusement Brain gives him a chance to kill him with a sword he enchanted to trade one person's life for another. In other words if Gray stabs anyone with it then that person will die in Juvia's place and she will live. Brain turns himself into a horrible and powerful monster. He and Gray fight but Brain proves to be too powerful. Realizing that he can't kill Brain and unable to let Juvia die, Gray stabs himself with the sword. This act proves that Gray's love for Juvia is real so Brain is destroyed, the forest is freed, Gray's wound heals, the exceeds become human again, and Erza's power is restored but Juvia still appears to be dead. Gray holds her and tearfully apologies to Juvia, telling her that he loves her and has always loved her and will never love any woman but her. Juiva wakes up and the curse on her is finally broken. Later Juvia is reunited with her family, she and Gray get married, and they all live happily ever after.
On the side, Gray's friend Natsu is trying to convince his sweetheart Lucy to marry him but she doesn't think he's romantic enough. Just something to add a little humor to the story.