For example, this fan wiki page contains a list of some work that is now in the public domain.
As I understand it, it is legal to sell these works and even new works based on these. What is not so clear for me is what happens when you port over that work to another medium/art form.
It seems, from what I've read that it would be okay to make an extended version of the cartoon or to shoot a live version with actors, i.e. drop the cartoon element.
However, could one legally create and distribute a short video game based on these movies? A potential problem is that while the work has fallen into public domain, the characters are trademarked.
But if so, why is that not an issue if one produces an extended cut of the original?
Otherwise, under which criteria could this be done?