I am completing my MPH right now (between M2/M3) and am writing my thesis on Outcomes of injury using EMS factors. There are state and national databases you can data-mine to complete epidemiological studies related to EMS if that suits your interests, and really, in my experience, the key is talking to some EM folks at your institution who have their MPH or who are in your area of interest to get ideas/mentorship. Really any sort of system-based study could be fair game, as long as you link that system to public health and your MPH program requirements. I'm sure you could also use surveys, investigate costs, or link policy to outcomes if you frame it the right way. Just my 2 cents, best of luck!