This is my theory:
With the end of the first book, another appears, with other versions of the worlds we already know: Enchanted Forest, Wonderland, Neverland, etc. After discovering the existence of these alternative worlds, Henry, now 18, wanting to write his own story, leaves Storybrooke to explore these new places "where the happy ending isn't written".
After spending a few years exploring these new realms, he travels to the Enchanted Forest, where he meets Cinderella on the way to the ball. Henry accompanies Cinderella to the ball, where Prince Charming falls in love with her. The story twist is that Cinderella ends up falling in love with Henry, while Prince Charming is the real villain.
After being rejected by Cinderella, the prince, who is a tyrannical leader that wants to rule all Enchanted Forest, demands that both Cinderella and Henry being beheaded in his front. With the help of Cinderella's fairy godmother and princess Tiana, Henry start to fight against the Prince's regime. Maybe the fairy godmother is Tiger Lily, but I still have no idea why she's in this new Enchanted Forest.
Seeing an opportunity to return to high society, Lady Tremaine supports Prince Charming and helps him in his hunt for Cinderella and Henry. The prince is also being helped by an old friend, Alice, who in this version is a worlds traveler who has fled from his homeland, Victorian England to Wonderland. Alice and Drizella fall in love, but are prevented by Lady Tremaine, who can't bear the idea of her daughter falling in love with another woman. Secretly the two flee together.
With the help of other kingdoms, Henry defeats the Prince, who is quickly thrown into prison. With the blessing of the Fairy Godmother, Henry marries Cinderella and the two are crowned king and queen. The years go by and Cinderella gives birth to Lucy.
Wanting revenge, Lady Tremaine kills a fairy godmother and steals her wand. With a magic wand in hand, Lady Tremaine becomes a threat to all realms. Desperate, Henry returns to Storybrooke and asks Hook, Regina, and Rumple for help, who are completely surprised, because for them it has only pass a few years.They follow Henry back to the new Enchanted Forest, but it's all in vain. Obsessed with revenge against Cinderella, who blames for everything that has gone wrong in her life, Lady Tremaine sacrifices Anastasia, the only daughter left on her side, to cast a mighty curse that will destroy the happy endings of all, the Dark Curse. Thanks to a Tiger Lily spell, Lucy becomes the only person immune to the curse. Armed only with her father's storybook, Lucy is given the mission to help Henry break the new curse and be reunite once again with his true love.
The curse is cast and Lucy wakes up in the Land Without Magic. In the new world, she is the daughter of Jacinda, a single mother who shares the apartment with a friend, Tiana. Instead of creating a whole city, the curse creates a new neighborhood in Seattle, Hyperion Heights. The residents of Seattle are kept out of the new neighborhood while their residents are barred from leaving.The curse gives the book a new look, but the stories are kept intact. It doesn't take to long to Lucy find her father, who she knows willn't remember her. Lucy drags Henry to her house, leaving Jacinda embarrassed, her daughter brought a stranger into their home.
Henry then becomes friends with Jacinda and her daughter. Henry feels responsible for giving the girl these fantasies, because he is the author of her book. Thanks to the curse's fake memories, Henry thinks that grew up in a small town in Maine. As a child, a teacher who he now barely remembers the name gave him a fairytale book that for a while he thought that its stories were real and that everyone in his hometown were the cursed characters in the book. After leaving Storybrooke, he used his childhood memories to write a book about a character named Emma, that would have been the mother he never had. This book is the book that Lucy has now. With the book and Lucy at his side, Henry begins to realize that there is something weird in Hyperion Heights.