The word dissertation is generally equated with the formal written work of significant depth and length which students must submit in order to graduate with an advanced degree. The topic for this kind of dissertation is, theoretically, something that has never been researched before, though of course it does require scholarly source material. In a less formal sense, a dissertation is any lengthy piece of writing. I'm going to assume you are working on the latter--perhaps a final analysis paper on some aspect of Wide Sargasso Sea by Jean Rhys.
Because this novel is a kind of prequel to Jane Eyre (written by Charlotte Bronte in 1847), one point of comparison is certainly how faithful--or unfaithful--this novel is to the original. Several of the characters, including both the protagonist (Antoinette Mason) and the antagonist (the man who is unnamed, also known as Edward Rochester), are also found in Jane Eyre. Consider making a comparison of these characters in both novels. We know that Antoinette is portrayed as "crazy" in Jane Eyre; does what happens to her in Sargasso effectively explain this transformation? Discuss her dramatic change from sympathetic protagonist in this story to a raving lunatic bent on destruction (which obviously makes her an antagonist) in Jane Eyre. Lots of potential writing ideas comparing these two works and the characters in them.
Another aspect of the story is the strange dichotomy between the beautiful setting and the awful things that happen here. Jamaica is a land of stunning beauty; however, it is also a land of wildness and death. It is an Eden
gone wild. The paths were overgrown and a smell of dead flowers mixed with the fresh living smell.
When Rochester arrives on the island from England, he describes it this way:
Everything is too much.... Too much blue, too much purple, too much green. The flowers too red, the mountains too high, the hills too near.
This analogy to the biblical Garden of Eden is interesting, as it appears to be the perfect place but it does contain evil.
Racial tensions also deserve analysis in this novel. Despite the beauty of the island, it is full of paranoia, suspicion, bitterness, hatred, and worse. Because Jamaica has been recently freed from Colonization, tensions between the freed slaves (blacks) and whites (symbolic of British oppression) are hot and violent. Consider the relationships between colors and classes in this novel, including the unjust condemnation of Antoinette by the blacks as a rich, privileged woman simply because she is white.
Consider comparing the writing styles of Rhys and Bronte. Why do you suppose Rhys used the characters Bronte created but did not write in a style that is in any way similar to Bronte's. Even more challenging: find the similarities between these two disparate writing styles.
Perhaps you could analyze all the different kinds of isolation which are present in this novel. This isolation occurs because of race, class, geography, societal expectations, sanity and insanity, and communication or lack of communication. Most of the tragic things that happen in this story can be traced to this theme.
Antoinette has three dreams at three different times in her life. Think about how the dreams she has foreshadow events which happen later in the novel.
If none of these appeal to you, think about any aspect of the novel which interested you and think about why it had some kind of impact on you. It's easier to write when you feel something about the subject.