Just getting into a PhD program is tough. It might be a good idea to narrow it down to some locations you like and then check the profs in the field and see if you're interested in what they're doing. As you know, you need to be part of a team to get in . Profs like you to have an area of creativity to explore, but they are thrilled if someone can bring some skills and interest in their current projects. Does that make sense? Are you totally bored with your MSc area? Another approach might be to ask this, "What does the world need right now that you can find out more about or help with?" People go into health sciences because they want to help the slow process of solving problems. Where can your areas of expertise help? How about Ebola? (Just kidding, you're not a specialist in infectious diseases -- but boy that's a bad one.)