I think that the Ontario Line would be better to go up Dufferin. Its already the highest ridership bus route that is not planned to get higher order transit.
It will be a bit of a sharp turn at the Ex, but its doable.
Im not really in favour in mirroring regional rail lines with other modes of transit. With proper signaling, EMU's etc, you can just as easily convert the regional system to have more local transit. And you are not dedicating 2 tracks to the subway system, which means instead of say 3 regional rail tracks and two subway tracks, you have 5 regional rail tracks, which allows for more optimization of regional services along all 5 tracks.