what is a good doctorate degree choice of study if you already have an MBA?

I know lots of MBA/JDs (juris doctorate—law)

Figure out what subjects you’re sufficiently passionate about to devote 3 to 5 years of your life in study. A wise person defined graduate school as “that period of time where you study a subject you once loved and are fairly sure you’ll love again”; if you don’t care deeply about the subject, you probably won’t be able to put up with the academic BS long enough to get the doctorate in the first place. If you don’t know what you want to devote the next 3 to 5 years of your life to, you probably should take time to reflect, because the answer has as much to do with you as with the job market.

For that matter, what are you hoping to accomplish with a doctorate? Without real-world experience, the business world won’t take your doctorate or your MBA seriously—people with MBAs and lots of book learning but no experience are a dime a dozen—and if you’re doing it for yourself, the only criteria to worry about are what you want to study and whether you can afford to devote the necessary time. And the academic mindset pretty much requires single-minded devotion to the subject, because there are enough people with that single-minded devotion that universities can always hire enough professors from that pool, so people without that devotion get passed over. You’re polling relative strangers about what you should study—you’ll get chewed up and spit out by the academy.

What do you feel passionate about? What would you skip meals, give up nice cars, live in a dorm for 3 years, if only it meant you got to spend 16 hours a day studying it?