Whether Wonka is an origin story really depends. If the narrative details how Wonka developed his skills, personality, and motivations from the start, then yes. But if it jumps in later in his life, then probably not.
One possible fanfiction could be about Dipper and younger Wendy going on a mystery-solving adventure in the woods. Maybe they discover a hidden fairy glade that no one else in Gravity Falls has ever found. They have to work together to figure out the secrets of the glade, and in the process, their friendship deepens.
No. Willy Wonka is a fictional character from the novel 'Charlie and the Chocolate Factory' which was first published in 1964. Although the story is set in an imaginative world full of magic and wonder, it has no basis in real events from 1896 or any other time in that regard.