It starts in an alley. An angsty Italian gang creeps onstage in a “stylized prologue showing the restlessness of the youths.” It’s New York City in the Fifties, and, as the plot progresses, warring ethnic groups articulate their frustrations via song and dance. Children die preventable deaths; everyone sings; the audience thinks soberly about prejudice and peace. I am referring to a musical called “GANG BANG! (working title).” It will, of course, eventually become one of the most popular musicals of all time, known by a much sleeker name. But for now, it’s merely a fuzzy sketch of an unwritten...