During the FIFA World Cup tournaments, there's provision of a special Visa for fans to the host country or countries and such Visa expires immediately at the World Cup is over. So any fan who has that kind of Visa to the host country is considered an illegal migrant at the expiration of the tournament.
I don't think it is difficult to get tourist visa for US as for Canada I don't know.In here many of my friends have already gotten a tourist visa for 2 years,not for the World Cup but to enjoy traveling and seeing US,once they have done so they can apply again and it will be far more easy the next time.For the moment right now they don't ask stupid things,except the normal bureaucratic procedures which come down,who you are,your legal and marital status and what is the level of your income (to find out if you can afford to go there for tourism and not for illegal migration purposes).
So even when the time comes for 2026 World Cup they will make it easier than it is although for me it is easy enough right now for anyone to get a tourist visa from US until now.
I believe that during the championship the USA can simplify entry and exit for fans, as it was done for the World Championships in Qatar and Russia. In addition, all of Europe can travel to North America without visas, so I think that a lot of people will come. Air transportation between Canada and the US is excellent, so traveling between the cities will not be a problem. I think the biggest problem may be crime, especially in Mexico, but I think the fans will be guarded heavily.
I think that for fans the matches in Canada will be easy to attend.