It’s not so much new highway but upgrades to existing ones. MTO has had plans for things like the 400 widening, 401 widening to Kitchener, 401 widening in Durham, 403 / QEW widening in Burlington, etc.
A lot of the local highways are also over capacity these days and need upgrades too - 89 from Alliston to the 400, 9 from Orangeville to the 400, 12 around Beaverton, etc.
Regarding new corridors, I agree not too many are needed. I see 5 real new ones in the Golden Horseshoe, none particularly large, and beyond highway 6 from Hamilton to Guelph, I don’t think any are really particularly high priority:
1. 404 to highway 48. Doesn’t need to go to 12 like it was previously planned, but should fill the gap in the highway network by connecting to 48.
2. 406 extension to highway 58 - this area is growing quickly and this would improve connections to Port Colborne and south Welland.
3. 6 highway upgrade between Waterdown and the 401 - partly planned already but this should be upgraded to a RIRO Like 35 or 11.
4. The new Brantford-Cambridge highway.
5. Highway 6 extension to Caledonia. The upgrade to the airport planned right now is really probably fine for now, but there is a lot of growth ramping up in Haldimand which may make it necessary eventually. Maybe include a Hagersville bypass as well, but it only needs to be a freeway until Caledonia. It would actually likely be well received by the 6-nations as it would help their economic development to have better freeway connections.. they have not had issues with development on the north side of the Grand River either as the treaty on that side is not disputed like the south side is.