Me personally, I'm planning my trip next year in October to avoid the crowds and virtually guarantee that everything will be open and operational with few interuptions/glitches. We did the same thing when the original WWoHP opened, and it was far easier to enjoy the area in October than the horror stories about the lines and delays in late May/early June.
We missed New Fantasyland's opening last fall by just a couple of days. They were conducting soft openings the week before we arrived, but transitioned to cast member and VIP previews the week we were there, and had to look at the new area from afar. We had no expectation of getting into the area, so it wasn't so disappointing, but I was a bit bummed that we missed it by just a few days. With something as big as Harry Potter and the new Disney coaster, I would not even go within a month, give or take, of the projected openings early next summer.
I would never plan a trip over 200 miles around a projected attraction opening. Now if I lived in Florida, and had annual passes to all of the parks, I would stop by every day through the months of April, May, and June to try to get in during the soft openings, but since I don't I'm not planning my trip until I'm pretty sure all of the major new attractions will be open.
Ditto all of Russell's advice, of course.