I'm not sure about the Visa situation in the USA but I know taking Visa and traveling to America is a bit harder than other countries, but I'm sure they will change some rules for the world cup. In fact, during the world cup there should be some other rules for the people who come to that country to watch the games for example in Qatar they could make some special rules for visitors.
I also believe that the visa rules will be made more flexible, but the fans cannot count on that.
So, imagining that everything continues as it is and considering that the 2026 World Cup starts in the first week of June, then the fan will have, from now on, only three years to prepare, and he should mainly consider that he may have to wait longer than a year waiting for the consular interview.
If I'm not mistaken, you first need to fill out the DS-160 form that is found on the US Department of State website, then access the Visa Applicant Service Center website and pay a fee, and only then schedule the face-to-face visits for interview, collection of biometrics and photo.
With the US visa in hand, it is possible to obtain authorizations to also enter Mexico and Canada.