WestJet's B737's have 130 seats, whereas Air Canada's B737 MAX 8's are expected to have 16/153 layout (same as UA B752s), and given the lack of business demand, if anyone is able to make the route sustainable in the long run it'll be WestJet.
WestJet's current B737 transatlantic operations to DUB and GLA require a stop in St Johns/Halifax. There is (albeit limited) demand on St Johns - Dublin and Halifax - Glasgow given historical ties, though there would be no such demand for Belfast.
If WestJet were considering BFS (I'd be shocked if they weren't), they'd probably wait until 2019 which is when their B737 MAX 7's are being delivered, thus allowing BFS-YYZ non-stop.