Many Golden Age Superheroes fell into the Public Domain.

Particularly those that were neither DC or Marvel.

This website chronicles them.
http://pdsh.wikia.com/wiki/Public_Domain_Super_Heroes
Golden Age Comic Book Superheros aren't all it chronicles, but that was it's initial premise.

Some characters' names are under Trademark (sometimes only because of similarity to a different character's names).  But that only hinders the ability to put their name in the title or on the cover of the work.

So basically there are plenty of Comic Book Superheroes out there you don't have to be either Warner Bros, Disney, Sony or Fox to make a movie out of.  But unless a character is as Super Iconic as Sherlock Holmes or Dracula, major Studios would rather use characters they can have complete control over.

What I mainly want to talk about right now however is how many Female Superheroes the Golden Age had.  They may have been handled in ways unsatisfactory to a modern Feminist, but this myth that Female Characters are inherently less likely to sell does not really seem to have been as much of a problem back then.

Characters like Miss Masque, Miss America and Miss Victory.

One thing I've often noted in my mind is about Kid Sidekicks.  Most sidekicks are the same gender as their mentor, however there are many female sidekicks to Male Heroes.  But I couldn't think of any examples of a Superheroine with a male Sidekick, like society can't handle the idea of a woman being the mentor to a young man.

That was until this website made me aware of the Black Cat and Black Kitten.  So there is one known example of at least one Female Hero and Male Sidekick team.

There was an Australian bat themed Superheroine in the late 1940s called Vampire.

Legacy of The Masque is a web-series that uses PD Superheroes.  Here is it's Facebook page.  I'd very much like to see more people try similar projects.

Many Charleton and Fawcett characters fell into the Public Domain.  In fact it seems even many of their characters DC regularly uses are PD in their original forms, but you just can't use any post Golden Age elaborations of their lore (and they're Trademarked).  Even Captain Marvel is listed on this site.  (Black Adam isn't).  On the Superheroine subject that includes Mary Marvel, (but not Isis, she was a 70s creation).

Another character DC owns the later history of but is none the less in the Public Domain technically is Phantom Lady.  It is in the case of this character that we know DC may try to sue even if they don't have a case.  So it's definitely something to be careful about.

A lot of the Golden Age characters of Timely Comics (later Marvel Comics) fell into the Public Domain.  But not their big three, Captain America, Human Torch or Sub Mariner, and their major supporting characters.

One such character is Miss Fury.

One ongoing plotline was Marla's adoption of Darron, the abandoned son of Erica Von Kampf and Gary Hale(Marla's former fiancé). Marla was unaware of the toddler's parentage but, she rescued him in Brazil from the brutal guardianship of Diman Saraf, who planned to use him in a deadly experiment. She was initially denied custody of the boy because she was a single woman, which broke her heart but, her courage and heroism in the rescue eventually won over the authorities. Marla was then, for the duration of the series, an unmarried woman with a child: a rather progressive scenario for a 1940s adventure strip

But the most interesting Timely character to me is The Blonde Phantom.

This was a post war character introduced in 1947, and her involvement in some Espionage related story-lines makes me feel it would have been very logical to include her in Agent Carter.  I suspect Marvel Studios and Disney don't want to make the PD Superheroes of the Marvel Universe part of the insanely popular MCU because they want all the movie stars they make to be ones they can completely control.

Sometimes I suspect the Blonde Phantom's look may have been inspired by The Domino Lady, but that's probably just me.

Jean-Marc & Randy Lofficer choose to change the name of the Arsene Lupin supporting character The Blonde Lady (La Dame Blonde in French) to The Blonde Phantom for their translation, Arsene Lupin Vs Sherlock Holmes: The Blonde Phantom.  I haven't gotten to read the book yet so I don't know the significance of that decision, but I do know that they are aware of the Timely character's existence.  The story is set in 1904, so it's not possible to imagine it as exactly the same character, but they are fans of the Wold Newton Universe concept.  The original French Novel and it's oldest English translation are both pre-1923 so totally PD, but the Lofficer translation is not PD, it is published by Black Coat Press.