What Topics Are Suggested for Ph.D. Research?
It is an extremely broad question, and probably the wrong question, but I actually think the answer is pretty short. I'll just give you the advice that I was given (from multiple people), and the advice that most people I know would give:

http://matt.might.net/articles/phd-school-in-pictures/

In other words, you start with an interest, and that interest develops into a specialty. You deepen that interest, and then you expand a small amount on it in your PhD. Telling you what to do not only in some ways would defeat the purpose of you actually getting a PhD (i.e., you picking a specialty and pursuing it because you like it), but it says some pretty strong things about a PhD: that is, I personally think it's more helpful to think of a PhD not as the crowning achievement of your career, but as a good start, and since yours will almost certainly not be revolutionary (like, say Syntactic Structures), it is much more descriptive to explain the process than it is to tell you what specifically to do.

Good luck, though!