This comment isn't so much about the northern extension to York U. which is needed and beyond, which is questionable, but the soutern point at Union Station.
I always thought they should look into extending the Union platform both east and west then seperating the Univesity/Spadina line from the Yonge line. This would increase the capacity of the Union subway platform significantly and would make the headways on both the Yonge line and Univesity/Spadina line more manageable. This would increase capacity on both lines significantly if the headways were shortened even slightly. For the passengers who travel south on Yonge or South on University who wish to continue north on the other line, all they would have to do is walk down the same platform and catch the connecting train- no stairs to negotiate.
Maybe this plan would be impossible from an engineering standpoint- but I always felt that the two lines should be seperated. Union Station would then be served by 2 seperate subway lines, streetcars, buses and commuter trains, its capacity could increase because the subway platform could handle the excess traffic and the elongated platform wouldn't get as crowded as it does now.