Obviously he's not going to let thousands of people to sail in line after him. Even if he accepted, he would do as you said and tell the people to sail out wherever they want and do whatever they want. The Straw hat pirates seem to respect freedom above everything else, after all. The only times Luffy orders the others around is when Nami or Usopp try to keep him away from an interesting (=dangerous) place, when they try to interfere with his fight, or when they try to steal his meat. There's no other captain like that, and certainly a captain like that isn't suited to actively lead a huge fleet. He could accept the role, but I don't see him doing anything much with it, at least for now.