A doctoral candidate has to choose a topic out of the lot to begin with his research. The research is processed after the candidate chooses an interesting topic. Researching on a topic and writing a thesis on it is easier than choosing a topic to research. The candidate has to be farsighted to choose a topic, and shall pick a topic which can be researched.
Out of thousands of topics, there is virtually no topic that is left untouched. The researcher should keep in mind that he has to choose a topic which is not researched too much or too little. A too much researched topic will give a large number of matter for the candidate to study, whereas a too little researched topic will not give suffice matter to study and thus the matter in the paper would not be properly researched.
The first step to choose a topic is that it should be interesting. The candidate should pick up a topic which interests himself. There are great papers written in every field but the point is to find an interesting topic for themselves. While choosing a topic, one must ask himself if any strata of society or research community will be benefited from the research done. Why would your university allow you to proceed with the research on this particular topic?
The candidate should be ready with the review of recent research studies done on the selected topic to know what's coming on his way. He should be well aware of the literature existing on the chosen topic.
The most important thing to keep in mind while choosing a topic is that the research should have a closure. The research should allude at an engaging issue for the readers to ponder on it, with a methodology to resolve the issue. The issue might have been raised before and left unresolved. But your research should address it as well as provide the reader with the solution. Thus, not only your research topic should be exploratory but also the approach you use for investigating it.