Even without fans, staging games would require an extraordinary level of quarantining for everyone involved. Let's say you test every player and filter out anyone infected. Do they then all live together on a fully regulated compound? What about their families? Do you make sure nobody ever goes anywhere and comes in contact with anyone else? Are all games held in one city, or do they travel, and if so, how? Travel will become possible, but still adds complications,and charters solve some problems but not all. If in a single city, how many games can you realistically play each day? The NBA can play on high school gyms, many of which have regulation or near regulation courts that might require minimal modifications, so a single city could probably handle it. But baseball, football and soccer need big playing areas. A junior baseball diamond with 275 foot fences won't work for MLB and I doubt there any cities that have more than 2 full size baseball fields suitable for MLB.