I have a theory...
If a movie is really in the works, then a movie toy line at retail will definitely happen.
They could, theoretically, be tooling this castle for this line, but it may also be an investment for later.
A movie would likely have a very classic, iconic, looking Grayskull... but even if it didn't, slight modifications to the mold would be FAR cheaper than sculpting / tooling an entirely new one.
I believe that this castle will appear again if a movie line happens... maybe it will be a different color, with different accessories and features, but it will likely be the same base/shell.
I think that is why the castle isn't going to be $350 / $400.
That is also why I think it will definitely happen.
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That cardboard cut-out that had the shield on it! That would be SO cool in plastic!!