The problem is that the Midtown Line is not at Annette street, it's at the West Toronto Diamond and there's no room to build platforms there anymore, nor would there be any convenient way of accessing them from the surrounding neighbourhood.
To be clear, I'm not going to debate the merits of the ideas above at all (cost, convenience, time etc). Far too hypothetical for my liking at this juncture. There is no Ontario Line at all, yet; and none seriously contemplated west of Exhibition. Still.....
Indulge me, please.
Assuming one were envisioning running the O/L within the Kitchener Corridor above or below grade:
I will put in for the idea that a station at Dupont/Annette on roughly the original site does appear feasible at first blush (room for station and building).
The southern extent of my line is the Dupont underpass, and I've taken it north to where the space available for a platform gets ever smaller. You get a about 268M of potential platform/building, where the width is
generally in the range of 30M over what is mostly the footprint of the original station, between Old Weston Road and the curving track.
Here's an archival image of the original station building:
Credit offered in-line, taken from :
https://www.trha.ca/trha/history/stations/west-toronto-station-canadian-pacific-railway/
Total distance between a platform at the original site, and the mid-town line would be 80M (from the closest point) .
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Now, building a platform for the mid-town line is somewhat more constrained. I've assumed that would have to be at-grade with the current tracks.
If one draws a line from the Kitchener Corridor west to to the CP WB connection from Mactier to Galt, I think an 8M wide platform is viable, IF one can encroach on the tail end of Old Weston Road behind the City pumping station. (I'm assuming a northside only platform)
I get about 160M of length, assuming the platform is not extended across the tunnel; if it is then you're in the 230M platform length area before the Galt to Mactier West to North connection forms another barrier.
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None of which is to say that its a good idea, the only reason I put in for its feasibility is a selfish desire to rebuild the original heritage station, and to suggest, even in fantasy threads, one shouldn't
prematurely rule out what's possible.