What is the worst case scenario for a dislocated jaw?

Permanent. Worst case scenario is that it will stay dislocated for the rest of your life.
Broken. Worst case is that is is broken. It depends on how it was dislocated. Not infrequently bad temporomandibular joint arthritis could make your jaw lock open or shut. Sometimes medications to relax the muscles or oral steroids could take down the inflammation enough to get things moving again. Best bet is probably an oral surgeon.