In addition to his post at the Zurich Opera, Fabio Luisi is the principal conductor of the Danish National Symphony Orchestra and is succeeding Zubin Mehta as the music director of the Maggio Musicale Fiorentino. Earlier this decade, many observers had expected Mr. Luisi to become the next music director of the Met. He won over critics and audiences while filling in for an ailing James Levine in the midst of a new production of Wagner’s “Ring” cycle, and was named principal conductor in 2011. But Mr. Levine returned after two seasons away and resumed his post.