I wonder if there is anything more intense than the writing of a post graduate work sample. Given the fact that this is for a doctorate, I can only implore you to speak with professors and instructors whose opinion you really value. I only stress this to you before all else because the writing of a doctorate work is something that will be one of the most difficult challenges you will ever undertake in your time in the academy. It will be a task where you will research a mountain or summit of information, engage in multiple drafts of composition, perhaps reevaluate topic selection, and endure some bouts of frustration and stagnancy in the composition process. This is all prior to your defending the writing sample to the harsh glare of academic criticism. In relaying this, I think that being able to find advisors or individuals who can offer insight and guidance when your intellectual wheels become "stuck" or might veer on a track that might not be in your best academic interests will be of vital importance. Being able to speak to them about what can be composed on Tagore would be good.
I would also ask you to engage in some reflection about Tagore yourself. In the end, you, and no one else (we hope), will be writing this paper, this work. What is it that you find about Tagore so fascinating? To what element of his work or his composition would you be able to devote so much scholarly work and time? I think that the suggestion of any topic has to mesh with your own personal views about the topic. For example, Tagore was really quite radical in his depiction of women in his stories. I cannot think of any other writer, Indian or otherwise, that would have been able to come up with the characterization of Ratan from "The Postmaster" or the relationship between husbands and wives in "Punishment." This might be a potential topic to explore. Yet, in the end, this is going to be something upon which you will have to ruminate and reflect. Think well because the decision you make on this will be quite important and significant.