While I was in Santa Fe, our instructor Oliver Prezant proposed that opera is about the conflict between love and duty. This came up in discussions about Maometto II where both Maometto and Anna are torn between their love for each other and their duty to their two countries. In our version duty wins. In the version with the happy ending love wins.
So is this true? How many operas show this plot? Of our five in Santa Fe clearly this is the plot of Tosca. The title character is not the conflicted character. Here it is Cavaradossi who is torn between his love for Tosca and his duty to his fellow political partisans. In Tosca everyone loses.
In The Pearl Fishers the conflicted character is Zurga, the newly elected king of the pearl fishers, who is torn between his duty to his friend and his unrequited love for Leila. He comes to his senses in time.
Arabella is about jealousy. It's one of those unfortunate coincidence plots.
King Roger is about .... There is no love plot. I think.
La Traviata is a love vs duty plot. Alfredo has a duty to his family which he ignores in favor of his love for Violetta. Since the father cannot persuade his son to return to the family, he instead persuades his son's lover to turn Alfredo back to his duty. She knows the only thing that will work is to degrade herself in his eyes.
Cosi fan Tutte is about the girls' duty to their present fiancés versus their love for the new young men. Mozart/Da Ponte say that they will always choose love.
Carmen, everyone knows, is about the conflict between Don Jose's love for the utterly fascinating Carmen and his duty to his mother and Micaëla. The right Carmen can make us forgive him.
There are at least two conflicts in Rigoletto. Gilda experiences conflict between her duty to her father and her love for the Duke. But it is the second conflict that brings tragedy. Rigoletto fails in his duty to protect his daughter and gives in to his passion for revenge. It is a unique and wonderful opera.
Nothing is more clearly one of our plots than Aida. Radamès betrays his country for the sake of his love for Aida. Everything else is subplot.
Werther is another opera in our category. This time it is Charlotte who is conflicted between her duty to her promise to her mother and her love for Werther.
I like it. It isn't the only plot, but it covers a lot of operas.