Players and fans have plenty of time to create visas. But not every fan and player should get multiple visas, because cup format is made so, that qualification part will be in one country, but play-off and finals will be in US. Canada and Mexico will be presented by two stadium each, but US with 11 stadium all across country. So if you are a fan of a weak country (sorry for that) you probably dont have to get multiple visas

For players it will be much simpler. Like cruises. You get visa to US to board the cruise ship and go ashore to Mexico without Mexican visa. I suppose same system will be for fans also.

U see only one problem with USA - a lot of people gonna get visas "because of football cup", but some of them gonna stay in US as immigrants for ever, taking into consideration fact that they cant get visa there normally. Such things always happen with poor countries. There was even a case (dont remember if it was Olympic games or football cup), when plane with athletes landed, and during the event part of them disappeared, because they ran away from poverty and dictatorship in their homeland.