Possibly the most frequent approach to the dissertation is to study the work of a particular author or group of authors. A student might examine some aspect of the author's body of work that hasn't yet been explored. Or, if several authors are studied, the dissertation might compare and contrast how they each dealt with a specific issue. The goal is to uncover something new. Originality is one of the most essential factors in writing a good dissertation.
Dissertations often take genre as their focus. Genre means the type of literature, as in poetry, drama or fiction. The evolution of the short story is one example of a dissertation on genre. Another example might be the difference between a novel and a romance. Some students have pondered the issue of why a certain genre is successful during one time period but neglected during another. Genre topics for dissertations are extremely open-ended.
Studying the literature of a particular time period has always been popular. These dissertations often explore a train of thought or belief system that was fashionable during the time being studied. Another approach might be to challenge the accepted designation of time periods. An example of this kind of dissertation would question the accepted definition of "Modernism" and possibly suggest an alternative. In fact, questioning the status quo is often an effective dissertation strategy regardless of subject matter.
Sometimes new veins of research can be uncovered by mixing different academic disciplines together. For example, a study of classic literature's understanding of biology in the nineteenth century could be very revealing of attitudes about science. Philosophy, religion and literature are often co-mingled to produce fascinating dissertations. The trick is to build connections that are meaningful and build a foundation for more research. The readers on the committee must gain a new perspective from an interdisciplinary dissertation to make it worthwhile.
Don't think of any one category as a closed box. It's perfectly acceptable to integrate genre, author, time period studies and more. The priority is to craft writing that is interesting and original. You have to convince the dissertation committee that you have more to contribute to the academic discourse. Thinking outside of the box will get you at least halfway to that goal. Careful writing and patience will do the rest.