Rob,
Your airport options will probably depend on your destination in Europe. Some airports have frequent flights to some locations, but not to other popular spots in Europe. For example, you'll likely find good flight options from Toronto to Rome or Montreal to Paris. Air Canada has the greatest number of possibilities, however in some cases they transport passengers to a major hub (ie: Frankfurt) and then use one of their Star Alliance partners for the final leg.
Air Transat mostly operates seasonal flights to Europe, but switch to the sun destinations (Cuba, Dominican Republic, etc.) in the fall. Whether you'll be able to use them will depend on "when" in Fall 2015 you plan on travelling. They offer flights to some less visited locations such as Lamezia-Terme in southern Italy, if that's of interest. Also, A.T. typically only offers flights one day per week to some locations, so your travel schedule would have to be somewhat flexible.
WestJet is now offering seasonal flights (beginning 1 May, I believe) to Dublin or Glasgow from St. John's, Newfoundland. At the present time, they'll be using newer generation 737's on that route, but they have some 757's on order so will probably be using those on Europe routes in future. WJ offers connecting flights from many parts of Canada to St. John's. As I recall, they do fly to Arizona and points in the south, but under the current rules I don't think they're able to sell tickets originating there.
There are LOTS of possibilities!