I think an interesting question to ask (were I a policy maker) is: “How can we make larger portions of Etobicoke, Scarborough, North York, etc” more amenity-rich.
I suspect a lot of this is zoning, density, and built-form related. Many of the blank spots are low-density suburbs, many RD zoned. As a result, you can’t put amenities there, and as a business - why would you? Your addressable market is low.
Of note, a couple of those weak spots are being or are set to be addressed shortly.
Etobicoke City Centre; ie. Six points, will have a new Recreation facility, a new Library, likely a childcare centre, public space, employment etc.
Throw in walkable grocery and you're well on your way.
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One of those big holes in North York is the area around Downsview Park/Airport.
The redevelopment of said lands should allow for the missing amenities and a new road grid for easier, faster access to public transit and said amenities.
A couple of other pockets also have promise, the north Scarborough gap that's most noticeable is in/around Sheppard/Midland-Brimley.
With GO RER coming to the Stouffville land and considerable intensification proposed on the Kennedy side of the corridor, I would expect to see this spread along.
Not as conspicuous, but there's a bit of an issue in the Victoria Park/Finch area; which has the TCHC community of Chester Le nearby, and is slated for revitalization (redevelopment in TCHC-speak).
So there is potential there too.
Some areas could be helped along by more permissive/as-of-right zoning; others by better transit; while a few require larger-scale investments including new roads; or possibly City-led assemblies to address subdivisions that turn their backs on major roads.
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Something not addressed in this piece that is important is whether the 15-minute connection is remotely pleasant.
Its one thing to say there's a bus stop or a supermarket within a 15 minute walk; its another to admit that walk is along a tree-less, narrow sidewalk, in the sweltering heat of July ; or sans wind-break in a blustering January.
This is a second key area the City must address, insuring that making that walk is vaguely appealing, or at least not downright unpleasant.