Yeah, the only way the North Toronto subdivision is getting any type of GO service is if the CP freight traffic is diverted onto another line. The only real candidate for that is the CN York Sub, which would require a) an extension westward to meet up with CP's line in Milton, b) a reconfiguration of the tracks around Oshawa to allow CP trains to switch onto CN tracks there, and c) a co-share agreement between CN and CP to use CN tracks for CP freight.
Logistically speaking, it probably won't happen, which means GO service on the North Toronto Sub probably won't happen, unless CP goes belly up or GO spends billions for new tracks along the corridor (which, given the space confinements, would be very tough).