I don't think Apple will release a 512GB option this March. Ya never know, but I wouldn't bet on it. I believe they'll add other things, not the least of which is the rumored bezel-less 10.5" iPad pro along with faster chips. I'm wondering if Apple will move to a usb-C port instead of the lightning port as well as getting rid of the audio jack like in the new iPhone 7.
Very excited for this iPad pro update.