Still going off-topic here, please feel free to move this discussion to another thread if you see fit.
Since I was only 7 years old when the GTA Task force and Anne Golden were a thing, I did some reading up. I was confused about what you were getting at for a moment; Golden/Rae was trying to expand metro, while Harris essentially forced the amalgamation of the existing Metro Toronto essentially castrating any power the future metro would've had.
I still think the idea is worth re-investigating given that the former Metro has outgrown its boundaries. There's currently a mess that inter-city transit is with 7 different agencies running overlapping and staggered service in the GTA (8 including Burlington) and competing and sometimes conflicting transit and growth management strategies across the region. Services have the potential to be run less expensively
The Greater Toronto (or New Metro Toronto) would work well under the Regional Municipality model with local issues governed at the borough/city level, but overarching goals and direction managed at the regional level. Waterloo Region functions quite well under this model ensuring local matters like by-law and recreation are are handled by the cities while regional priorities such as transportation and long-term planning and oversight are handled by the region.
I can envision many Scarberians and Etobicokites who would be far more pleased to have a local mayor from their "city" they complain to as opposed to coming into city/metro hall and trying to be heard by the larger council.