I'd rather just see the 15" rMBP get a bit thicker so that it can have more variety of I/O and stay at the ceiling for battery capacity.
Does that also apply if it’s two batteries, both physically removable, from the outside? Just curious. SWMBO and I travel with can lights for diving, big batteries but none individually over the limit.
Btw, newer Lenovo T4x0 models have a very nice dual-battery configuration, which allows you to swap the battery without shutting down.