LONDON—Emma Raducanu’s astonishing U.S. Open triumph could lead to a pot of gold worth around 20 million pounds ($27.7 million) over the next two years and that may be just the start, according to sports marketing experts. The 18-year-old Briton was almost unheard before reaching the fourth round at this year’s Wimbledon, having earned around $40,000 since her senior debut three years ago. After becoming Britain’s first female Grand Slam champion for 44 years and the new golden girl of women’s tennis, Raducanu’s earning potential is set to enter the stratosphere. The $2.5 million she picked up in prize money for beating fellow teenager Leylah Fernandez in a final that captivated the sporting world may soon seem like loose change. Raducanu’s mixed Romanian-Chinese heritage, a stunning game, and engaging personality make her “brand gold” according to one sports marketing expert. She is already a cover girl for British Vogue’s October …