Browsing these websites may help you identify a topic you are interested in researching.
Oxford Bibliographies in Criminology provides a list of criminal justice-related topics. Each topic entry defines/introduces the problem and provides a list of suggested readings and key articles on that topic.
You can search Credo Reference to get an overview of a topic and current research related to the topic.
Sage Research Methods Cases provides examples of criminal justice research projects. These examples could help you choose a methodology for your research proposal. Search by "criminal justice" generally to browse some research projects or search with a specific criminal justice-related topic.