The easiest way to find a research topic for the application of JML is to explore some related work that you can find in publishing websites such as: Springer, Elsevier, IEEE and so on.
Anyway, here's a research paper that might be helpful:
You have to do a rigrous literature survey to find the area suitable for you as JML is not limited to single branch.... you can take the help from latest research articles...
a) Well, you may do a case study where you may apply JML to verify a commercial application. This process will expose the strengths and weakness of JML. Previously, researchers notably Erik Poll, Néstor Cataño and Marieke Huisman applied JML to verify some protocols but a comprehensive case study is missing. I myself is planning to do this case study in future:)
b) You may think about to generate JML specifications from Java code by performing static analysis.
c) JML does not support to specify concurrent programs. You may think to extend JML so that it may provide support to reason concurrency.