I agree with many of your guesses/statements above but this one below is almost for sure incorrect.
Originally Posted by mapezzul
The drive shaft and half shafts will be much lighter than now (more on that in the future), as will the rear diff, wheels and suspension components. Decreasing unsprung weight as has been mentioned before will be a huge target and the new designs of the CF roof.
The engine will have much more torque and this will require stronger components in much of the driveline. Without very expensive composites stronger generally means heavier.
Wheels: Existing wheels are quite light. New car may get 19" standard and 20" optional. From the base 18" wheels this would also likely be a weight increase.
Suspension: Without advanced and very expensive use of composites it will be challenging to save much weight compared to the existing cars forged aluminum parts.
The existing CF roof itself is very light and not even a structural component (which contributes to its already low weight).
I agree BMW is going to press very hard to control, manage and likely to reduce the weight of the new M3. It just is most likely not going to come from the areas you have mentioned.