I think having anyone take on the Joker in this 'arc' of the Batman legacy (the Nolan movies) will be the exact opposite of whe hype behind The Dark Knight.
You can't tell me that a good chunk saw the movie mainly because Ledger died. I wanted to see the movie before, but the idea that this was one of Ledger's last movies added to the need to see it.
To recast the role could have a backlash that would have a completely opposite effect to what we saw with TDK.
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