Antonio Citterio and partner Patricia Viel found themselves in an enviable position. Not long after completing the Bulgari Hotel in Milan in 2004, they were approached to design luxurious accommodations for another high-end chain just a few blocks away. If the first reflected the Milan-based architects’ signature elegance and refined palette, the next—a new outpost for the Mandarin Oriental—would show off a more playful, and nostalgic, side.