I asked the Stihl rep at the local dealership's open house this spring if a larger blower like a 650 or 700 or something along those lines were possible in the future, and he said absolutely not. Stihl felt that going larger than the 600 was not the right direction for the company to move because the machine would have to be larger and heavier, matching or exceeding the competition, so it would negate one of their advantages they have over the competition right now. Plus the EPA is making things ever harder for companies to design new and better products that can improve much over what's available currently.
That said, I wouldn't be surprised to see just such a blower come out. Hopefully if it does, it'll be a 2 stroke.