After reading some criticisms about A Streetcar Named Desire, you will want to gather details that you think apply to and support the topic you have chosen. Certainly, there are many approaches that you can take to Wiliams's play, and there is a list of further topics to explore here on enotes. (See the site below for these).
But, to make a suggestion, you may wish to write an analysis of Blanche DuBois, an examination of her character as it follows the symbolic meaning of her enigmatic entrance speech:
[BLANCHE with faintly hysterical humor]: They told me to take a streetcar named Desire, and then transfer to one called Cemeteries, and ride six blocks and then get off at--Elysian Fields!
In a symbolic sense this statement chronicles the life of Blanche DuBois who has had a past of much carnal desire, she has encountered death, and she has gotten off at--Elysian Fields by the end of the sixth scene. There is an essay cited below which you can read in order to generate some of your own ideas on this thesis if you like it. Also, see the character analysis, as well.
I am going to explain this in a way that makes thesis statements so much easier for my students. I hope it helps. Think of three to five main points that you want to make (depending on how long the paper needs to be). Each of these points will end up being your topic sentences. Now you must connect these points in one sentence- your thesis statement. For example, if you wanted to write about how Blanche's insanity helped her avoid the miserable life that her sister led, your could begin by writing your topic sentences such as...
1. It is obvious to the reader that Blanche is incapable of functioning in the role of a wife.
2. Because Blanche is unable of functioning as a wife, she escapes the cruelty that her sister Stella endures on a long term bais as Stanley's wife.
3. Because Stella cannot escape marriage like Blanche can, she is just is much of a victim in the end.
A thesis that would tie all of these together would be: Although it seems like Blanche is the ultimate victim in the play, Stella is just as much a victim because she has no mental escape from her abusive marriage.