I just did a little measuring. The vertical part with the ports is 6 mm. The shaped part below is 5mm. And no it is not true that connectors inside are significantly bigger.Look at a a logicboard from both sides. They need those 6mm and that is it. For Type-A USB and TB/DP, type-c would need less still.
The 5mm are filled mostly with battery. If you shrink the logicbaord like in the Macbook, thin out the keyboard and make the heat sink thinner but deeper. You could even cool it almost the same. The battery would need to shrink and a newer even skinnier battery would be needed. Shrink the belly down to 2mm instead of 5 and you got a noticeably skinnier MBP.
It is doable especially if they dump the dGPU on the 15" and not try to go all wedgy. The heatsink design could also be optimized, imo those should be bigger. Like the Razer blade mostly has a bigger heat sink which gives it the extra cooling power. It is the same height just deeper and bigger. They should have made room for that and reduce the max necessary rpm and noise. Worth the bit of extra space imo. Anyway they could do it now thinner and better to come out about the same level.
Newer cpus don't really need 95Wh for decent battery life. Displays also get more efficient which reduces the Wh needed. Batteries have been unchanged in the rMBP but they get better every year. A new pack would probably need less space this year than this 2 year old one does for similar performance.
It is all doable even with Type-A USB but I seriously doubt they would do a redsign without switching to Type-C all round. I just don't see it happening this time around. Except that if they drop the dGPU they will do it this generation or not for a few, because once they switch to Maxwell and later HBM 16nm GPUs, Iris Pro won't look good enough in comparison. If they go that route then they would release something else like a 17" along with shrinking the 15", I think. If that was likely we would hear rumors. There are none so there will be just the same old with new chips in all likelihood.