IT’S NO SECRET THAT, from the beginning, Takashi Murakami harbored ambitions to become a superstar in the global art scene. And, of course, he achieved this goal in short order: In 2005, Murakami completed his Superflat trilogy—begun in 2000 with “Superflat” and followed by “Coloriage” in 2002—with “Little Boy,” exhibited at the Japan Society in New York. The show was succeeded by a 2007 retrospective at the Museum of Contemporary Art, Los Angeles, which subsequently toured the world, and, three years later, by “Murakami Versailles.” What is left for him to accomplish? The largest