Regarding the extensions of the existing 400-Series Highways, allow me to offer my input.
69/400: Finishing the link to Sudbury will finally create a proper north-south link between Southern Ontario and the rest of Canada.
401: The extension to Detroit (the Windsor-Essex Parkway) is a must and should have been done 50 years ago.
404: A small extension would be nice as it represents a route on the eastern side of Lake Simcoe. It will surely be extended if the Bradford Bypass is approved.
406: Widen the existing un-twinned part, but I'm not sure if an extension is justified.
407: I think it can be extended to the 35/115, especially since it will be tolled and owned by the province.
410: As someone who lives outside the GTA, I can tell you that the 410 is just as much a long-distance freeway as it is as a commuter route. Extending it to Orangeville is a good idea in my opinion as it would help travelers who are traveling to Collingwood, Blue Mountain, Wasaga Beach and other areas. Building it further than that is probably overkill.
17/417: The Trans-Canada Highway should have been built as a freeway across our country. If the US Taxpayers could afford the entire Interstate Highway system, surely Canadians could cough up enough cash for one east-west freeway. Twinning the segment between Sault St. Marie and North Bay might be good for the long-term, but I can't see it getting upgraded much more than that.
420: Extending it to connect with the Thorold tunnel and Highway 406 would take a lot of pressure off the QEW in St. Catherine's and Niagara Falls. Perhaps a further extension might be possible if the Mid-Peninsula Highway is approved.
427: Canada's second busiest highway just seems to come to an abrupt halt north of the 407. A small extension might be justified. It will surely be extended if the Bradford Bypass is approved.