Well, I would like to see what will happen to countries like the USA and Canada that require visas for South American countries, if there are so many teams that are going to be on this side of the world, it is something that can lead to many problems, unless that these countries give temporary visas due to the number of tickets they have, but fans who are from these countries will give them tickets? I know that even Mexico requires a visa, at least for Venezuelans, something that seems absurd to me, but hey with emigration problems and all that (which is off topic, but I'll touch on it anyway) so what will happen to that=?
When talked about world cup, the visa situation is easier. You give them a visa just for the duration of world cup, usually something that is like 40 days long or 45 days long, and you accept them if they have a ticket. That's very easy to get Visa during that period, normally that would be at least an obvious reason to go to USA during that period.
However, you do need to provide a proof, hotel ticket, game ticket, something like that, and it would make sense. This obviously changes if you have nothing to show for and absolutely no reason to be there or if you are a criminal lol. I obviously couldn't go, I can't even afford the ticket to go to USA, literally just the ticket itself, nothing else, I wouldn't be able to buy a plane ticket that goes to New York.
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.