I can't see a major change to the MBP for at least a year or two. What would they do? It's already about as thin as they can get it. Maybe they'll scrape off a few more millimeters. The frame around the display is already quite thin.
Eventually they'll switch to OLEDs of some sort. That will make the display thinner.
It can't get much thinner or there won't be room for the ports which are of fixed size.
Eventually they'll make SSD standard which might free up a little room. Maybe they'll drop the optical drive at the same time. Again, maybe it will get a little thinner and lighter.
Probably they'll add better cameras, microphones and speakers, but that won't change the case.
Maybe the touch pad will get a little larger and gain some new features.