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I'm not a lawyer either, if copyright is expired, you're free to make a derivative work. Although, you certainly have to be sure you don't derive from a derivative work whose copyright has not expired.

Trademark issues will be tricky, there is a concept of fair use in trademarks, and it seems to me, if you can sell a copy of the film, you must be able to use elements of the film to sell it. If you can create and distribute derivative works, you must be able to use elements of the works to sell them. But maybe we'll just have to have white boxes with 3rd party Mickey (as seen in Steamboat Willie) movies, and boring movie posters too. I expect to see a stream of lawsuits until clear rulings are available.