For your dissertation on Germinal and L'Assommoir by Émile Zola, you might consider comparing the plight of the working class, a topic quite prevalent in both novels.

Comparisons and discussion points can be made about the main characters in each novel: Étienne Lantier in Germinal and Gervaise Macquart from L'Assommoir. Both undergo a harrowing journey to better their stations and wealth amid the social and economic circumstances of the day, and both succumb to a cruel fate, despite their efforts. Zola's greatest strength is his ability to show the working class toiling upward without progress, much like the efforts of Sisyphus.

One thesis discussion can look specifically at the decision to end both novels without the main characters succeeding or achieving their goals. Both protagonists fail to grow beyond their circumstances. What does this tell us about our world, our desire for riches and prosperity? What were the dreams of each character, and how were they lost within their lifetimes? When did each character give up? What circumstance or chain of events led them to abandon their goals?

With a study of each main character, and how their journeys parallel (or even differ from) each other, you could also include the places they appear in other Zola novels. Gervaise Macquart, for instance, makes an appearance in La Fortune des Rougon. Her daughter Nana continues on in the novel of the same name, which you could argue serves as another example of the constancy of toiling and the inability to improve one's station in society. Equally, Étienne Lantier, from Germinal also makes an appearance in L'Assommoir.

Another topic of discussion is Zola's depiction of the plight of women in both novels. You might show how Étienne Lantier's lover Catherine has a similar plight to Gervaise Macquart. Catherine works in the mines and has an abusive lover; she leaves him for Étienne Lantier. Although this should be a step up for her, eventually her circumstances worsen, leading to her entrapment in the mine, which offers rich symbolism.

Equally, Gervaise Macquart works as a laundress and is abandoned by Lantier. She eventually marries Coupeau, which seems fortuitous at the start, with her dreams of opening her own business within reach. However, Coupeau is injured, ushering in more hardship for Gervaise Macquart. What is Zola attempting to say about the role of women in society and business or about their ability to gain independence and prosperity?

Further assessment could be made by including other female characters in Zola's novels, like Gervaise Macquart's daughter Nana or Denise Baudu from Ladies' Paradise.

I think that both ideas sound like good ones. Given that Zola was a Naturalist, you could focus the dissertation on how he followed the characteristics of Naturalism in each of the novels.

Both novels contain elements of the everyday working class, both involve addictions typical to the working class of this period (alcoholism), the settings are common- as well as the characters themselves, and the fall of the characters is based upon the ever present ideology that nature is stronger than man.

I think that for a dissertation, keeping such a focused theme would harm the strength of the paper (especially since you are only focusing on two novels). If you look at the characteristics of Naturalism within the paper, you will have a lot more information as to prove your thesis more solidly.

If you do plan on looking into one of these ideas, a broader focus would be the personification of elements in both novels. The mine in Germinal is repeatedly described as a beast swallowing the men.

Once you decide, you can check out the Naturalism page on eNotes. It offers a great tool to understanding the elements of Naturalism.