W. K. Lis
Superstar
If the Midtown Corridor becomes an eastwards rerouting of the Milton Line, then there are only a handful of stations I would say are absolutely essential: Jane/Runnymede, Dupont/Spadina, and Crosstown. Jane/Runnymede for redevelopment, Dupont/Spadina to shift commuter traffic onto the University subway line, and Crosstown to interchange with Line 5 or the Ontario Line. I'm skeptical that the North Toronto station would be able to handle all downtown-bound traffic on the line.
Other important stations would be between Symington and Dufferin (serving either the Dupont/Dufferin neighbourhood or to interchange with the Barrie Line), the Summerhill/North Toronto stop, Ellesmere/Warden (lots of redevelopment potential), and a Markham Park-and-Ride somewhere south of the 407. It could also be extended to the future Pickering Airp- just kidding.
I think service to Peterborough is more appropriate for Via HFR than GO, since Peterborough is large and distant enough that encouraging Toronto-bound commuting just isn't economically viable.
With the Galleria development at Dupont/Dufferin, that neighbourhood would be a good draw for a GO station nearby. See link.