A dissertation in English Literature is no easy matter as it involves a dedicated study of at least 2 to 3 years. The theory part, the collection of material, the discussion with your guide and your writing skills will decide the quality and coherence of the dissertation. The first thing is to select a topic for which you have a genuine interest.
As your question partly answers your query and reveals your interest is in Literature than in ELT, Critical theory or Linguistics, it would be better to discuss the issue broadly with your prospective guide and choose from Poetry, Fiction, Comparative Literature, Socio linguistics, Drama, Indian writing in English or other topics which are less explored and need to be researched to provide new perspectives or themes.