In that case, write it. Not as a dissertation of course, but as an article or small book.
My dissertation held my attention because it was about something I was passionately interested in: why it was that in the seventeenth century, people started talking about there being one truth for science and another for art, a subsidiary assertion being that normal language was degraded, but that the language of science (mathematics) was pure....and that therefore, science was by definition "true."
Unravelling all that took me about five very interesting years looking into history of science, history of mathematics, philosophy of language, etc. The result was that I answered my question to my satisfaction but that it rendered me basically unemployable because most of the stuff I talked about was unintelligible to English depts. anywhere. (I did get a tenure track job, but it was based on my experience as a tech writer, not on my diss.) I never regretted writing the diss.
So, if you're passionately interested in working through some question, do it. It's a hard kind of fun, but it's fun. And you will never regret it.
Joanna
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