Seeing as this is the new DRL thread (perhaps a merger is in order?), I did come up with an idea: Why not use Transit City rolling stock instead of traditional subway EMUs? The key advantage of this is that it would use overhead wires rather than a third rail for power, which could provide far more flexibility in the routing. While the line should be as separated from other traffic as much as possible for obvious reasons, the flexibility could make extensions more feasible.
For example, if it is to continue north to Eglinton, it could piggyback on the Millwood bridge across the DVP rather than building a whole new structure. Likewise, it could continue north to Eglinton via either Laird or Overlea/Don Mills, and perhaps branch off of the Eglinton-Crosstown line. This could allow a single seat from McCowan to downtown without overcrowding the Yonge line!