But the Harry Potter theme is not going to be stale anytime soon. It is a proven commodity and has reached a Star Wars type level. The avatar theme is already very stale and there is no guarantee that the sequels for Avatar will regain even close to the same interest.
You wanna know Avatar's fate probably is: Jurassic Park or the Matrix. Revolutionary 1st movies with mediocre sequels.
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You can bet that Iger and Co have seen advance ideas/materials relating to the sequels and are convinced that Cameron is onto a good thing. This adds more pressure to make the sequels a success. It is a gamble, as these things always are, but it could pay off big time.