The big news for fashion lovers visiting Paris is the reopening of the 100,000-square-foot Dior flagship at 30 Avenue Montaigne, where Christian Dior started the brand in the 1950s. It now encompasses everything from haute couture salons and high-jewelry workshops to showrooms for housewares and children’s clothing—and even one special guest suite. But the biggest thrill for visitors is the adjacent Galerie Dior, which is a museum focused on the history of Dior—and by extension, the last 70 year...