The Wife of Bath's Prologue and Tale make for some fairly random reading. She has a tough time staying on topic. She makes frequent breaks to other stories such as Ovid's story of Midas. She also breaks her prologue narration quite a bit on various tangents. At times her tangents are about her relationships and other times it's as if she is attempting to back up her experience with evidence from other writings.
Regardless of whether you focus on her lengthy prologue or her actual tale, both focus on the relationships between men and women. Her prologue focuses on her attempts to rule over her husbands. She does this through sexual dominance, sexual rewards, rumors, lies, verbal abuse, etc. You name it, the Wife attempts to exert her will over her five husbands.
The Wife's actual tale is along a similar vein. It starts off with a knight raping a woman and being sent off on a quest of knowledge to find out what women most want. If he returns with the correct answer, his life will be spared. He returns with an old hag that provides him with the correct answer -- women most want to be in charge of the man in their life. The hag's answer was not free of charge though. In exchange for the answer, the knight had to marry her. The knight is not happy about this, and the hag presents him with another choice. She can be old and ugly, but faithful and loving or she can be young and beautiful but unfaithful. The knight tells her that he cannot choose, and tells her to choose. Because the choice was given to her, and she was given "the power," she chooses to be beautiful and faithful and loving.
For a thesis, I would choose something that focuses on how the Wife views the meaning of marriage and the roles of men and women within that marriage. For example: Although the Wife of Bath appears to support marriage and the union of man and woman, she more likely craves a partner that she can exert her will over.
Although historically the man is portrayed as the head of a marriage, the Wife of Bath shows how that is not the case.
The Wife of Bath has an incredibly negative view of men as illustrated by her lack of honesty toward her husbands, her negative portrayal of male friars in her tale, her portrayal of King Arthur submitting to the decisions of his queen, and the knight's inability to make a final decision.
While is appears that the Wife of Bath's prologue and tale both indicate that women most want to rule over only their husbands, her stories actually indicate that women want complete sovereignty.