The Captain Marvel/Superman case was about copyright not trademark. A trademark grants a company the legal right to exploit a name (or brand if you will) in a certain category. Others can use the name in different categories, but for the categories granted by a trademark it is the sole purview of the trademark holder. Trademarks have nothing to do with copyrights or plagiarism, it is a legal right to conduct business in a certain area using a certain name. Public/open domain pertains to copyrights mostly too. Part of Edgar Rice Burroughs Tarzan books are in the public domain because their copyrights expired. I can publish an edition of those books and even create new stories featuring the characters because of that. But ERB Estates still holds the trademark to the name Tarzan for published material, so I cannot sell my works under the name Tarzan even though they feature that character because that domain belongs to the trademark holder.
This is what caused the row between ERB estates and Dynamite a few years back. Even though the copyright on the material is expired and it is in public domain, the trademarks are still active and ERB acted to defend their trademarks. They can't stop people from publishing editions of ERB material in the public domain, they can however still control the name Tarzan for all marketing and commercial use and prevent others from using it.
Hasbro has a trademark on the name Bumblebee for action figures. DC can still use the character in comics, movies, what have you, but Hasbro holds the rights to sell and market toys with the name Bumblebee. That is what a trade mark is. A right to control a name and how it is used in a certain field of business. Hasbro was granted those rights for Bumblebee.
-M