Unless and /or until WestJet gets the go-ahead from the powers that be (i.e Ottawa) it is a moot point.
We do not have an Open Skies agreement with too many countries, and I have not seen anything on any bi-lateral giving WestJet authority to do any scheduled transAtlantic flying.
So, unless there is a way to do it as a subcontractor to somebody else, or in the form of charters, I won't see it happen soon.
In the case of charters there is plenty of seasonal demand to smaller European centers that do not get a lot of attention (because it does not warrant widebody service), but in that case it would depend on feed to/from other Canadian cities to either YHZ or YYT.