It’s an early-October afternoon in the penthouse offices of Rap Genius. The sixth floor taken up by the 4-year-old startup—in a modern glass building on the waterfront in Brooklyn’s Williamsburg neighborhood—is sparsely decorated. Other than a large mural of Instagram pics on one wall, it looks almost empty. But there’s a balcony with the Rap Genius logo on the railing and a panoramic view of Manhattan. It’s a good place to dream of empire-building, of conquering the old ways splayed out across the East River.