Generally speaking, opera companies see themselves as having two functions: offering new or unusual work, and offering productions of the classics to please or appease their core audience. The Washington National Opera has taken to opening its seasons with a “bucket list” opera in a serviceable production: This year, it’s “La Traviata,” one of Verdi’s most beloved and beautiful operas, in a new production by Francesca Zambello. How popular is this approach? This is a co-production not only with Zambello’s Glimmerglass Festival but also with the Atlanta Opera, the Seattle Opera and Indiana University.