The physical attributes needed for soccer are quite different from football or basketball. I don't think any football or basketball players would be good at soccer even if they tried.

Nope, not true at all. The average NFL CB makes the best athletes in soccer or rugby look like furniture movers, even in drills designed for soccer. Same for the majority NBA PGs. There is a lot of overlap in team sports for specific types of athleticism. The average pro you see on TV could usually beat the best player in your hometown at whatever sport they played. And not just beat, but beat badly. There are NFL DTs that can do 360 dunks and run a 40 in 4.6sec (at 300lbs) which is faster than your average pro striker in soccer.

Lebron as a goalie? Or hell half the nba...