Dissertation Idea #1: Health
Take some bigass panel datasets like MEPS and AHA. Test every theory about rising U.S. healthcare spending. If you take long enough to write it you can see the effects of the new regulatory regime and subsidies.
If you want to do something really useful, build a dataset about the regulatory regime in each state and use this to analyze the effects of state-level policies. This would be an extremely useful dataset to many researchers, but could also take a very, very long time sifting through state legal codes to build.