Specialized reference works can help with "brainstorming" potential topics. They provide brief introductions to, for example, theological loci, and major theological figures, movements, and events. Some of these works include brief bibliographies at the end of articles. There are comparable types of reference works available for related disciplines such as biblical theology, church history, and philosophy. Here are a few recommended theological dictionaries.
After gleaning information and perspective from these types of reference works, a next step may be to find relevant chapters or portions of chapters in systematic theologies.
New Dictionary of Theology: Historical and Systematic (Second Edition) by 
Global Dictionary of Theology: A Resource for the Worldwide Church by 
Evangelical Dictionary of Theology by 
*Dictionary for Theological Interpretation of the Bible by