As someone who finds the 13" just slightly too small and the 15" just slightly too big I propose the following:
-12" for those who need ultra portability (11" is just too small to do anything. Get a tablet if you're in this crowd). Budget model; ULV or dual processors, integrated graphics. The new Air (typical student, everyday browser laptop).
-14" for those like me that want a steady compromise of power and portability, and physical screen real estate. Quad core and discrete graphics.
-16" for the power users: tons of screen estate, large footprint and internal volume for the best hardware. Fastest CPUs and GPUs.
These would all be in the current MBP chassis meaning uniform (cube) body shapes. I hate the teardrop shape of the Air. Drop the Air and Pro lines and SIMPLY have the 12", 14", and 16" MacBooks. Retina on all models. Fusion drives on all models to optimize performance/storage needs.
How does this not seem like the best solution? It screams elegance IMO.