After reading chapter 697, i believe everyone has now no doubts that Law hides something and that it's highly possible he uses luffy to accomplish his ulterior motives. I 've heard 2 theories about this subject:
1. Law wants to get Dofla out of the way and desires his place in the underworld. Plus, if dofla quits being sichibukai, then he could even take the dirty place in the goverment and everything this includes. (Selling slaves, have much affilitation in the world gov, black market and all these being covered in exchange for his title). He uses Luffy in order to have an advantage in case something goes wrong.
2. I 'll make this short cause i m running out of time. He wants to aid luffy and he tries to manipulate dofla to make luffy the pirate king. This is truely far fetched and baseless, however this may be affilitated to Law's past.
Personally, I believe 1st theory is the one most likely to be true.
Sorry for bad voc,i 've got no time.I NEED YOUR CONSTRUCTIVE CRITICISM AND YOUR OPINIONS.