Tight End would be about the only spot that Lebron could play. Certainly fun to think about, but I can't imagine Lebron being able to take some of the shots that he would. Being that big in the NFL just makes you a big target.
It certainly would put a new spin on receivers going against corners for a jump ball. "Hey Lebron, I'm going to throw it 11 feet in the air, you go get it"