In that case, MTO need only twin 17 from MB/ON to Highway 71, 11-17 from Shabaqua Corners to Nipigon, and
possibly the gap of Sudbury SW Bypass between City Road 55 and Highway 69, finish 400 extension, add a second left-turn lane from 11-17SE to 63NE (in addition to what sonysnob suggested), construct Timmins Bypass, lower the speed limit on 11 around Temagami to 80 kph with serious enforcement, (edit) realign 17 around Montreal River, and leave the rest as is then.
9 items in total for Northern Ontario
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Extend 417 out to the CFB and the freeway 7 out to Perth, twin the 62 bridge from Belleville to Prince Edward Island (yes, that's an island county in Ontario, lol, in case anyone wonders), reconfigure 401/62 and 401/37 interchanges to eliminate weaving on top of widening from 4 to 6 lanes, reconstruct 401 outside of Napanee while widening, redo 401/"38" interchange in Kingston, and widen 401 to 6 lanes through the East Region. It'll be a surplus if QC's willing to widen its A20 from ON/QC to A20-A30 interchange. Since Legault's a suburbanist, it's now or never.
8 items in total (one of them a big one) for Eastern Ontario
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Widen 7-8 up to Stratford, construct Kitchener-Guelph Link, upgrade Hanlon Parkway, construct Morriston-Puslinch Bypass, convert the rest of 6 between Guelph and Hamilton into a freeway, widen 26 and bypass towns along the way, add passing lanes for 6 and for 10, twin 3 in Essex County, widen 401 from 4 to 6 lanes. That's all I can think of for Southwestern Ontario, excluding the Gordie Howe Bridge.
9 items in total (one of them a big one) for Southwestern Ontario
Edit: Add to the list Highway 24 Cambridge Bypass and widen it to 4 lanes between there and Brantford, making it 10 items.
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Widen 11 from 4 to 6 lanes and get rid of RIRO configuration altogether, and bypass the 90-degree turn at Gravenhurst. Eliminate the at-grade intersections as deemed fit.
Upgrade the RIRO section of 35/115. Continue work to widen Lindsay Bypass. Extend the freeway 7 past Highway 28. Widen 401 from 4 to 6 lanes past Cobourg (Exit 474).
5 items in total (2 of them big ones)
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GTHA's situation warrants a dedicated comment in the Ontario Highway thread.