Some welcome thoughts here about the research agenda from a scholar at a an earlier stage. I think formulating an idea of one's agenda as an advanced graduate student could be helpful, even if it ends up changing a bit later. Formulating an agenda is different from coming up with a dissertation topic. The topic is one piece of the agenda, not the entire enchilada. Of course, you can't work on the entire agenda at once.
I never wanted to be a specialist on a single author, someone who devotes an entire career to Milton or Twain, or even Lorca. The general tendency is for the agenda to expand over the years, to encompass more material.