Phoenix could have the greatest Joker performance of all time and won't help this concept imho.
The entire concept behind the Joker is this insane criminal whose background and motivations are so fluid and ever changing you can't fully grasp his intent. Making a movie centered around a character who is supposed to be unfocused will make an unfocused mess of a movie.
When he is playing off of OTHERS though, he becomes this element of chaos to the story that can leave the audience guessing and the story moving. That's his gold spot.