Hmmm, interesting thread.
If I had to do a PhD, I think I would research something to do with outcomes of arthroscopic surgery vs conservative management in managing femoroacetabular impingement (FAI) - research is currently being done into this (and rates of arthroscopic hip surgery have greatly increased in recent years), but there's no conclusive evidence yet to say that either surgical management or conservative management is the gold standard

(As an aside: Interestingly enough, the government actually revised the MBS (Medicare Benefit Schedule) items for hip arthroscopy to limit the items to treatment of non-FAI conditions).
Perhaps otherwise something to do with the incidence and outcomes of the condition (FAI) in general.
Along the lines of this, I wouldn't mind looking into developing more specific assessment tools for assessment of hip injuries - currently, we have a fair few highly
sensitive tools, but nothing that lets you make a definitive decision about what a person is actually experiencing. Compare this to say, knees, where we have pretty highly specific tests for things such as knee ligament integrity, etc.
I'm sure as my degree continues I'll find other things I'm interested in though haha. Don't see myself doing a PhD at the moment in my career, but maybe that will change once I've been practicing for a few years