Justin has to tread carefully on this one. There are plenty of women in his cabinet who like prostitution to be illegal, I bet. So if the Liberals are told to re-write the law it could be full blown illegal. The NDP maybe the only party in Canada to legalize prostitution.
I think you can write off the NDP as far as being a contender. At least, both the NDP and the Liberals are on the way down.
Jagmeet can't make up his mind whether he supports Sikh independence/Khalistan movement. He does not renounce his support for Sikh separatism in India, and is vague on the use of force by those separatists in India for an independent Khalistan. In any case, any person wanting to be Canadian PM can not advocate the breakup of a friendly and democratic country; it's interfering in the internal affairs of a foreign country. It would be like France officially endorsing Quebec separatism, which they never did. The other problem he has is his visible ethnic [and non standard Sikh]headdress, that will turn off a lot of Quebec NDP voters, into the hands of the Liberals in Quebec. I would also think that a lot of the non-Sikh voters of Indian descent who have some attachment to their land of origin, would steer away from Jagmeet and the NDP, with many going Conservative.
At the same time, Trudeau and his government's popularity have taken a dive in the polls lately, as a result of his disastrous India visit. There was plenty of press (and a huge amount in India) about the blatant selfie self-promoting narcisism of Trudeau, dressed in various Indian dress, and mimicking religious rites that he is most certainly not a follower of. He also spent very little time meeting with local government and industry people, looking as though he was thoroughly enjoying his vacation at the expense of the Canadian taxpayer. The Conservatives were recently leading the Liberals by 8 points. Trudeau's Chinese visit was also a disaster, as the anticipated breakthrough in Chinese trade did not materialize, with the Chinese PM Li Ke Qiang looking visibly pissed off after his meeting with Trudeau, in what resulted in him completely wasting his time.
Trudeau's populism has gone too far, with the 'peoplekind' gaffe the most recent public relations fiasco. So looks like the Conservatives are making a comeback despite themselves, and despite the blandness of Scheer. The Liberals, in the meantime, will not move on repealing C-36, as it's a political hot potato.