Consider the objectives from multiple angles. The topic needs to satisfy a question, resolve an ambiguity or controversy, or fill a gap in the area of study to fulfill the academic requirements to earn the doctorate. Other objectives may come into play too. A doctoral thesis or dissertation may be edited or revised or excerpted in part to appear in a journal. Portions of research may fulfill post-doctoral study or the requirements of residency programs. Decide how much the topic must accomplish to narrow the choices.
Relevant study defines the superlative doctorate topic. While any chosen matter must pertain to the contemporary state of the discipline, finding a topic that is relevant to the future of a discipline may give the final work an edge it wouldn't have otherwise. Since a dissertation may take several years to complete, relevance must be a long-term consideration. The topic must have enough flexibility to what is relevant now and in a few years.
The development of original thought and writing the dissertation itself already requires intense work. Diligent research into the availability of research may seem arduous, but it's necessary. A doctoral topic that lends itself to wholly original work is nearly impossible, simply because academia requires substantiation. A thorough discovery of available research will eliminate thin topics. Some topics may require considerable independent research experiments. If this is the case, consider the feasibility of conducting these experiments in a timely and relevant manner. A thesis that rests on independent experimentation may excel academically and jumpstart a career, but it may also destroy years of hard work if conducted poorly.
Since the thesis or dissertation very well becomes the center of a student's life, it's reasonable that the topic may influence a career for decades. Questions that pertain to the desired career direction are an important part of approaching the doctoral topic. From initial discovery through research and completion, the doctoral topic very well becomes a lifetime milestone.