Most of these changes mentioned by others have to do with the short-term and wouldn't really affect the doctrine. Long-term, serious doctrinal changes are perhaps much harder to predict or to speculate about. I see that some of those short-term changes tend to be cosmetic, or mere adjustments to particular issues.
In the long term, the Watchtower organization will have to change dramatically. Those guys who think that the Catholic Church changed just because it failed to enforce Bible principles delude themselves. Organizations have to change if they are to survive, and those guys have been able to dodge the bullets for 2,000 years.
Under its present circumstances, I don't think the Watchtower would remain a viable religion for most people. Maybe we won't be there to see the change, but I am sure that, just as the Watchtower changed so dramatically that Charles Taze Russell wouldn't recognize the organization he founded, in the future the organization will be such that a witness today wouldn't recognize it, either.
I think the "overlapping generation" teaching is one of the long-term changes. They just couldn't keep that. They will simply continue to change whatever needs changing if the organization is to exist.