Metrolinx already owns multiple sets of rail in the ravine including the not-yet-used Don Branch so we can use that for a future expanded GO service.
An interesting engineering problem is connecting future ravine stations to transit hubs above. Long/multiple escalators, gondolas, elevators, etc.
But much easier to do it to Don Branch and the Richmond Hill lines, than to the median of Don Valley Parkway -- there's some room to actually build platforms in some parts of the Don Branch (inactive) and/or the Richmond Hill Line (active).