The 400 needs a ton of work up to Barrie, the traffic on it is approaching the need for 10 lanes in some spots, but almost the entire thing is going to be hitting 100,000 AADT soon, which is generally the trigger for a widening to 8 lanes. Highway 11 needs work up to Orillia too, its approaching traffic saturation for a 4 lane road (generally around 50,000 cars daily) and it is extra unsafe as its only a RIRO. It needs to be upgraded to a full 6 lane expressway soon.
The 404 is going to need some widening work up towards Newmarket soon as well. MTO is planning to add an HOV lane all the way up to Green Lane in the near future.
The 403 through Hamilton needs a lot of work too. Adding an HOV to Highway 6 and widening it to 6 lanes through to Highway 6N would work. It needs an extra lane through Burlington and Oakville too, but I don't see that coming any time soon.
My understanding is that MTO wants the next big part of the 401 to be widened in Eastern Ontario to be between Trenton and Belleville. Its only around 40,000 AADT, but its pretty old and needs to be rebuilt, so they are planning to do it to 6 lane specifications.
The 401 in London is busy, but far from capacity. It peaks around 68,000 AADT near the 403, but drops to around 50,000 AADT through london. That is barely enough for a regular 6 lane road. You need to be north of 100,000 AADT to get an eigth lane.