There is no fixed rule of choosing a research topic but you should understand some basic principles while selecting a topic for writing the dissertation. It should be flexible enough, if you need to implement any new idea then flexibility is very important and it should have a base of real case studies. You should be confident enough so that the reader can also feel confident and all the arguments should have some basic facts.
When you are selecting a topic for writing the dissertation on criminology you should acquire some knowledge of practical issues of this topic because this subject is dynamic in nature so you should also prefer dynamic topics for writing the dissertation on this subject.