I recently made a radical proposal of public domain rules; It's inspired by GNU software licenses. It goes like this:

1. Anyone can use anything that is in the public domain.

2. Any creation that uses elements from the public domain is also, automatically, in the public domain.

3. Activate retroactively: When the first book in a series (for example) gets into the public domain, then the whole series (and franchise) becomes public domain.

(3) depends on what the initial rule is for something to get into the public domain.

P.S: It's a thought experiment, not an actual "let's implement it now!" thing.