In order to get here, we simply remove Justin Trudeau. Perhaps he takes a different career path and does not enter politics, or perhaps he is an MP but does not run for the Liberal leadership. Regardless, to have an NDP victory, we need to have another person as Liberal leader.
For my money, the best bet is Bob Rae, and that he broke his promise to not run for the full leadership after becoming interim. Rae has too much baggage (especially if he adds to it by snatching the leadership after saying he wouldn't) and would not connect the same way Trudeau did.
As such, the "story of the election" would simply see the NDP maintain it's lead, and not bleed support away to the Liberals. In fact, the opposite should happen, with Liberals going NDP to stop Harper from getting another term.
There are three provinces where the Liberals can expect to do "well". I use that term with caution, I don't mean they'll gain seats, or even hold seats. I mean they "won't be annihilated". These provinces are BC, Manitoba, and Nova Scotia. All 3 have reasons to oppose Harper (because he's Harper and needs to go) but all 3 also have concerns about the NDP (because they are the NDP and scary). One problem, though, is BC has another alternative; the Greens, and as such, much of that Liberal vote, will tilt towards the Green Party.
Going Province by Province...
Despite what I said, the Liberals won't win any seats here. Their strength is their weakness. The fact that people in BC want "a 3rd option." Without a good Liberal leader, more votes would bleed off to the Greens. So much that I can see the Green Party doing very well. As you see on the map, I've given them 6 seats, but in reality, they might have only taken 2 on Vancouver Island, with the other 2 going NDP, and the 2 Mainland seats going Tory.
Here we have the NDP sweeping New Brunswick. While that may seem crazy, the reality is that the vote distribution within the province allows for this. You'll notice the NDP has been adjusted down slightly in NS, but not by much, and has been adjusted in Newfoundland as well. Part of the reason for the latter is that the increase for the Liberals in the province was tied, partly, to Trudeau. (Also included are the final result totals)
Not much to say here. The Liberals do alright in Winnipeg, while the NDP does rather well by comparison.
The NDP had quite a concentration of vote in Edmonton, and as a result, would have been able to win a fair number of seats in the city had they retained their lead over the Liberals.
Here we have the NDP sweeping the province with only the Tories really in place to stop them.
We round off the projection with Ontario.
In the end we are left with a small but stable NDP majority, a strong Conservative opposition, a very weak Liberal party, a 2 seat Bloc Quebecois, and a 6 seat Green Party that finally has a caucus too large to fit into a single hybrid sedan.