Since the 2011 NDP win I posted earlier certainly wouldn't last until 2015, here's my guess on the following election. For this election, I used the averages for all federal polls in November and then used the UBC Forecaster to see how many seats each party would win, after I split the Conservative votes into those for the PCs and the Alliance. So it's all mathematical!
Layton passes away and the party chooses Nycole Turmel as interim Prime Minister until the party elects Thomas Mulcair as its leader. Because of all this revolving door of Prime Ministers, the government is able to survive for some time.
Mulcair calls an election for November 2013, hoping for a win after receiving rave reviews in parliament, and hoping that new Liberal leader Justin Trudeau's honeymoon will come to an end. Instead, the Liberals win a majority government, with the NDP forming the official opposition.
In a shock, both the Alliance and PCs collapse, with Canadians being weary of voting for new Alliance leader Jason Kenney, seen as too right-wing, while the PCs lose votes due to scandal involving former PC MPs Pamela Wallin and Mike Duffy, both elected in 2011.