Graphic design has evolved tremendously within the past 50 years and even within the past decade so when asked what the future of graphic design is, I am unsure, but I can say that it will continue to expand greatly.
Due to the ever-changing field of technology, I think that graphic design will change and it will grow due to the demand for it. Tools used for graphic design are getting complicated and trained designers are very valuable to companies needing them since graphic design isn't hand done any longer calling for a growth in graphic design programs in colleges. Plus, since it is so computer based today, graphic design appeals to more than artist, it appeals to those educated in computers and software.
I believe that within the next 10 or 20 years graphic design will still be computer/software-based with abilities that we can't even imagine today. Just look at what the creation and advancement of Adobe Photoshop program has done in the graphic design field, and it hasn't even been around that long. Graphic design will flourish and take over what we know as advertising today.