Concerning A Streetcar Named Desire, the answer to your question depends on whether you're referring to the plot and the fates of the characters, or to the play as a work of art.

You might see the close of the play as positive for Stanley.  His secret is safe.  He will get away with rape.  His life can continue as it has.  Audiences certainly should not see this as positive, though.

Other than that, I don't know that there's anything else positive about the plot and the way the play comes out.  Blanche is on the way to the mental hospital.  She has not been rescued, her sister doesn't believe her, and her illusions have been destroyed.  A possible exception to this is that Blanche still hangs on.  She is quite good at coping.  She views the conclusion as if she is being carted off--rescued--by another gentlemen.  The play is certainly not predominately about Blanche's perseverance, however.  She loses. 

As a work of art, however, the conclusion of the play presents a realistic, modern or postmodern ending.  It does not end happily.  Truth is often meaningless in our world, people get away with crimes all of the time, and society is not an accepting and pretty place.  Many stories do not end happily.  "Right" often does not come out ahead. 

Not that there is anything virtuous or "right" about Blanche.  She is messed up like all humans, as well as every other character in the play.  This, too, elevates the play as a work of art.