In reality, the NBA is the only league with a legitimate shot at working. The roster sizes allow for everyone to be in one location, no travel necessary. Once in the bubble and cleared, they're good. The problem comes when you introduce the need to travel, which the MLB, NFL, and NCAA will have. In theory, you could break these leagues down into smaller bubbles and have several scattered throughout the country, but it would require a great deal of planning and testing. NCAA probably would be harder to pull off because you'd have to allow these athletes to do classwork, but I suppose they could do that online.