Thank you waterloowarrior. You put my idea into something that actually sounds like a foundation for policy. I think this is what Ontario needs. It's achievable, practical, and a solution to Ontario's regional issues.
Going based on swimmer_spe's proposed 7 subprovinces (modified slightly based on my own thoughts) I drew this map:
Here's a close up on southern Ontario:
-Ottawa/Capital subprovince (RED) includes Ottawa, western third of Prescott-Russell (geographic county of Russell), and the municipalities of North Grenville, Beckwith, Carleton Place, Mississippi Mills, Arnprior, and McNab/Braeside. This accurately captures all of Ottawa's exurbs and leaves the truly rural areas outside it. Could serve as a strong foundation to keep Ottawa's exurban influence from spilling beyond that border.
-Eastern Ontario subprovince (PINK), covers the remaining (non-Capital subprovince) areas of SDG, LG, Lanark, Renfrew counties, and all the counties eastwards to and including Northumberland & Peterborough. Predominately rural with Kingston as the only real city.
-Central Ontario subprovince (BLUE): Muskoka, most of Parry Sound District, Haliburton, Kawartha Lakes, Simcoe, plus Brock Township in Durham region (this township is WAY too rural and isolated from Toronto to be part of the GTA province--it really shouldn't be in Durham to begin with...) Very rural, mostly cottage country, Barrie the only real city. Would likely have tourism as its main industry. Also a very socially conservative area.
-GTHA subprovince (GREEN): Toronto, Hamilton, Halton, Peel, York, Durham-minus-Brock. A city-state with great potential to become a highly intensified urban region
-SW Ontario subprovince (ORANGE): Grey, Dufferin, Wellington, Waterloo, Brant, Niagara and everything westward. Probably the most viable of the southern subprovinces, in the sense that it would include a healthy mix of urban and rural, with a diverse economy.
-NE subprovince (beige): part of Parry Sound district, and all the NE districts, plus a small section of Kenora district (which oddly enough, literally extends from MB border to James Bay...)
-NW subprovince (purple): most of Kenora district, plus Rainy River & Thunder Bay districts
The biggest problem I see with this setup is Kingston. In many ways (culturally, socially, economically, etc.) Kingston is very much an island from its surrounding area. Politically it's very progressive, whereas the rest of the Eastern Ontario-sans-Ottawa is one of the most right-wing places in Canada.
I think there's actually some merit to having Kingston completely on its own as the 8th subprovince, all to itself. It's a truly unique situation. The Southwestern province also has cities that are much more progressive than the surrounding countryside, but there's multiple ones--Kitchener, London, Windsor, Niagara, etc. Whereas Kingston is really all on its own. Barring that, Kingston should be a 'special municipality' with greater autonomy but still within the Eastern subprovince.