I think one of the top priorities for Ontario has to be a northern London bypass.
The 401 from London to Woodstock is a very busy stretch and increasingly dangerous with the huge amount of transport traffic. The 402 was never suppose to take the route it did but rather was suppose to continue east from Strathroy about 1 km north of London and hook up with the 403 at Woodstock. The reason it didn't is because the wealthy northern Londoners didn't want it led by the Labbatt family.
Northern London is the fastest growing area of the city and to get to the 401 can take 30 to 45 minutes in rush hour as London has no urban freeways. I know of people who live in Northern London who actually take HWY #4 north up to Lucan, and then head east to Stratford/Kitchener to get to Toronto as it is faster and certainly a more relaxing drive. The 403/Woodstock interchnage is parallel to Northern London so you are effectively heading south to the 401 for 12km to head right back up north.
This would be the equivalent of someone living at NYC having to head down to the Gardiner and then back up the 427 when trying to get to Kitchener.
The province is already having to look at widening the stretch to 8 lanes which will be a very costly project as widening to 6 lane is relatively easy with the bridges/overpasses but widening to 8 means a reconstruction of every bridge/overpass from London to Woodstock. An extension would not only serve northern Londoners but also Strathroy commuters and would greatly relieve traffic on the 401. Many don't realize that just west of London, 45% of the traffic merges off to the 402 towards Sarnia including 40% of the transport traffic...........it has become a very busy road.