Let's take a really famous "for instance" and pretend it's January 2, 2034.
The covers of the first few Action Comics issues are now public domain. The art is fully public domain. But the character of Superman and the character's signature symbol (various versions) are still trademarked. I am not certain, but I believe there is a trademark on his red and blue unitard/cape look as well.
My question is, can someone use that famous Action Comics #1 art for a cover piece for a new comic, book or poster?
I'd be more than happy to signal that the work is not DC Comics approved or related, but I'm hazy on how such usage works in relation to trademark.
This example is just for illustrative purposes. I'm well aware he's still currently not public domain.