Been thinking about about the best way to extend the Ontario Line westward for a while now, and this is what I've come up with:
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You'd start from Exhibition, tunneling under Parkdale to hit a stop at King and Jameson, with another stop at Queen and Roncesvalles (a connection the Lakeshore West line would be great to have here, too). The line would turn north, tunneling under Roncesvalles with a stop somewhere in the middle of the neighbourhood (I've notionally put it at High Park Boulevard, but this could be changed - there isn't one super obvious location for a station here), then connecting with Line 2 and the Kitchener/UP Express trains at a renamed Bloor-Dundas station. You'd keep it underground heading north, turning left to hit a stop just south of Dundas and Keele, with another station somewhere in the block bounded by Dundas, St Johns, and Gilmour. The line would turn north at Jane, creating a new connection to the Milton Line, and from there it's a straight shot up Jane, terminating at Pioneer Village Station.
On Jane, I've tried to keep ~1k station spacing. The exceptions are between Eglinton and Lawrence where the midpoint is industrial land along the rail corridor, and between Lawrence and Wilson where the midpoint is nothin' but sweet sweet highway!! Otherwise I've got stations approximately every kilometer until the terminus to provide fast but local service to the dense communities along the corridor. By the time the line crosses the rail corridor near Weston Road, Jane Street is sufficiently wide enough to allow the line to run elevated. This remains true until the terminus, though a bridge would need to be built to cross Black Creek near York University. At Jane & Finch, for example, an elevated structure could be built straddling Finch Avenue, with stairs/elevators providing a quick transfer to the Line 6 platforms (although one of the existing farside platforms would need to be relocated, unless the station building is built over the entire intersection which...... feels like a bit much)
This alignment seems to me like a good option because it covers the most new ground, while also creating a number of useful connections. An extension up Dufferin Street would be great, but north of Eglinton there's no obvious place to go that isn't extremely close to Line 1. Similarly, an extension down the Queensway with a stop in the Humber Shores area, possibly terminating at Kipling, is what I previously thought the best option might be, but it'd be duplicating the Lakeshore West line for half the alignment, then running through a relatively low density area for the rest. And so, replacing the Jane LRT (which, given how Line 5 and 6 have gone, probably isn't going anywhere fast (((in every way)))) seems to me like the best course of action.
The biggest questions I have (besides some of the questionable station naming) are 1: where to transition the line from underground to elevated, and 2: whether or not the line should divert off of Jane to make a connection at Weston Station.
With respect to where the line should emerge from the tunnel, I think the road is reasonably wide enough north of around Alliance Avenue such that an elevated structure could be built without TOO much trouble? But it'd have pretty tight margins, just eyeballing Google Earth. Plus you've got the elevated Line 5 station at Eglinton to get past, so an elevated Line 3 station here would presumably have to be quite the structure. Though, I'm not familiar with the soil conditions in the area (what with the area being a floodplain), so tunneling might be even tougher, but I really can't speak to this.
As for a diversion to Weston Station, I'll admit it's damn tempting, and an additional station could be built between Eglinton Ave and Weston Station along Weston Road. But I don't see a way to do it without tunneling from just north of Eglinton through to Highway 400/Black Creek Drive. If it isn't possible to build the segment between Dundas and Eglinton elevated, the cost of keeping the tunnel boring machine in the ground might be small, but otherwise I'd imagine that the project cost would get a hell of a bump from the diversion (but again, I'm not that familiar with tunneling costs, so I'd love to be wrong about that)
But uhhh... yeah! Sorry for the essay, this mostly just started as map making practice, but I figured it'd be a good idea to dump my needlessly convoluted thoughts here