I don't think that there have been a lot of plants built anywhere in the last 20 years as the industry has been plagued with overcapacity. Toyota did open a plant in Woodstock, ON in 2008. And in 2012 the Toyota Cambridge plant had capacity increased by 50k to 200k/yr. Honda added its second plant in Alliston, ON in 1998. And they opened an engine plant more recently - but they have also closed some parts of the plant(s). So there have been some new plant(s) and additions to capacity here in Ontario in the last 20 years.
Arguably the Fremont Tesla plant isn't new - it is very old as it dates as far back as 1961 when it was built by GM, at least according to Wikipedia.