Only a part of the new building would be a museum. Other proposed uses to go along with the museum include event spaces, retail, and compatible offices, and possibly the re-location of the library branch in City Hall.
I fear that this cheap-ass city won't go for it, despite the need for a central civic museum. The museums that the city operates are small historic properties, like Mackenzie House or Colborne Lodge; the archives out on Spadina Road are a bit out of the way, and those rotating exhibits should be at Old City Hall. We don't even have anything on the par of Montreal's two civic museums (the semi-public McCord Museum and the city-run Pointe-à-Callière) or the amazing City Museum of New York.