The 2019 election is the Liberals' to lose. When was the last time a majority government got kicked out after one term at the federal level? It has to have been a century or more.
One-term majorities without a minority buffer on either side are incredibly rare. Bennett had one of those during the Great Depression, Mackenzie did it following Macdonald's Pacific scandal, and...I think that's it. Clark had a one-term minority in 1979, and Meighen did it in 1926 as part of the King-Byng affair.
(Also, to expand your question out a little, Harper only had his one majority, but it was preceded by two minorities, and Dief had a majority with minorities on either side.)
In other words, if history is any guide, only having one term in office requires either a massive crisis (like the Great Depression or a Quebec referendum), a constitutional crisis (King-Byng), or the country going back to a beloved leader. Harper is neither beloved nor returning, I don't *think* there are any constitutional crises on the horizon, and I don't get the sense that there's a critical mass of Quebecers (or people from any other province) clamouring to leave Canada. That leaves a Depression-like situation.
A financial crisis or meltdown isn't totally out of the question -- in fact, one may even be likely -- but how that would play out remains to be seen. Harper was able to ride the 2008 crash to reelection, so it's not like it automatically hurts the party in power, provided they can be seen as calming. If we go into 2019 with Trudeau, Bernier and Singh as the three main leaders, I think the Conservatives will spend the election playing defense over charges that they want to slash social services at a time when we need them most, and the NDP will be left on the sidelines. If it's someone with a less extreme face leading the CPC -- say Raitt, O'Toole, or Scheer -- they might be able to present themselves as a return to the "normalcy" of the Harper years, though that approach would certainly have its drawbacks. (And if it's O'Leary or Leitch, all bets are off about what the CPC's strategy is.)
I think Pedrito is right, though. The election is still two and a half years away, and trying to forecast what the key issues will be right now is foolish, if not impossible. The last election saw multiple shifts over the course of ten weeks, and there was no way of predicting most of those shifts at the start of the campaign. Anyone who says that the next election is definitely, 100% going one way or another shouldn't be trusted or listened to.
