Okay, to address the challenge more fully...
Like I say, I don't think the electoral reality in Canada would support a full-scale reversal on multiculturalism. You'd really need some sort of major sociopolitical earthquake to occur, one which pushes all the major buttons of latent assimilationist sentiment in Canada. (Sorry for the mixed metaphor)
So, have it be revealed, during a Liberal government, that the leaders of some particular cultural group have been diverting money intended for language-programs and dance clubs to something like, oh, let's say, clitoridectomy clinics(like I said, this has to be pretty extreme), and that these leaders had close ties with people in the government. This gives you something like the Sponsorship Scandal, only with about 100 times the degree of awfulness and public outrage. Maybe a secret e-mail emerges in which one of the community-leaders brags about how clitoridectomies are okay, because Canada is a multicultural country.
The Liberals get voted out by an angry public, the Conservatives ride the wave of outrage into Sussex Drive, and promptly announce that they are abandoning the horrible policy of multiculturalism, in favour of the more assimilationist "interculturalism"(this may or may not help them electorally in Quebec). But, as I wrote earlier, this doesn't entail much of a change in practice, as the government still views it as worthwhile to pander to cultrual groups. But it does become considerably less fashionable for Canadians to brag about how "multicultural" they are.