Attorney from the US here, working in the entertainment law field here, and who deals in copyright matters on the daily, which is, in fact, my favorite area of US law (even thought its hot garbage mess on the best of days).
In the US at least, the requirements for something to qualify for copyright protection are genuinely very minimal. In other countries, it may be more or less. If you're French, the law is generally massively in your favor as an artist even. No one does artist protection like the French, truly.
But in the US the only requirements are 1) originality of expression 2) fixed in a tangible medium 3) in one of 8 categories of works (literary, dramatic, pictorial/graphic/sculptural, choreography/pantomime, music, sound recording, motion pictures/audiovisual, archtecture). If your work, or parts of your work, meet all three you "own" a copyright to the work or those parts of your work.
So actually there is a high degree of protection affordable to fanfiction and fanarts, outside if the limited elements from canon that someone else holds the copyright to.
For clarity though, an idea is not copyrightable. It's the original expression of that ide which is. For example, an idea is "coffee shop meet cute". The expresion that is original is what an author does with that idea, the plot and the story beats, the dialogue and the descriptions. You know you've seen or read various expressions of this idea across fanworks and fandoms to know it can be developed in a myriad different ways that are each unique. That uniqueness is the part of a story that would always qualify for protection under copyright law, regardless of the names and physical descriptions of the characters in the story.
DC might own batman and superman, but if you write a coffee shop meet-cute au fanfic about those characters, your original story and plot elememts, and especially, the particular syntax and dictionary of your story is highly protectable. That is your originality of expression in a literay work. It's what sets your fic apart from every other cofee shop meet cute written in every fandom under the sun, probably, including any other that may exist for Bruce and Clark.
See 50 Shades of Gray, notorious for having started its first life as Twilight fanfic. Not my fandom, but im sure its not the only kink fic that existed in that fandom. And clearly, the story had sufficiently original elements to it to be able to stand on its own and be a commercial sucess as a copyrightable work with some judicious filing away of specific canon elements.
Now, if you make something truly derivative of canon (think an different looking superman or batman suit), that would be a grayer area, beacuse one of Supes defining traits is that he has the suit with the S shield on it. When you get too close to something made by someone else, its harder to separate your originality from someone elses. You can't really "file the serial numbers" off a Superman suit too easily after all, without modifying the artwork significantly to where its no longer a "superman" suit. That isn't to say there aren't or couldn't be protectable elements in the artwork, it's just that they'd be so entrenched with the canon elements that a court would almost certainly consider the work as a whole derivative. Saying something is derivative does not void it of all originally of expression, it's just that it steps on the toes of someone else's rights to their elements. This is the reason fanart generally cannot be used commercially. The idea behind that is that your taking a bite out of someone else's money pie and that isn't fair in the eyes of the law. It's not to say that there was no originalty of expression in your work.
But that cuts both ways, which is why authors don't want you sending them detailed ideas for a story, and why studios return unsolicited screenplay unread. If DC came up with their own original Bat & Supes meetcute story, the last thing they want to contend with is a fanfic author trying to sue them for stealing their story elememts. If they don't look at anything fanmade, they can assert that as part of their defense. For the love of your fandom, don't send your fanfics to the authors! It's the fastest way to make sure that idea never sees an official light of day.
All that said, RPF is actually a very intersting area, because there is no copyrightable canon elemts you're using. Traits of a human person are not copyrightable. Mother nature cannot hold copyrights in the eyes of US law. RPF, especially sport rpf, is not tyically pulling from a "canon", unless it's pulling from music videos or other creative work, but from real life traits and events of people. The reason these cannot be commercially marketed is because they'd infringe on things like someone's right of publicity and potentially could be considered libel, if the person was fussed enough about the content, but only if you were trying to posit the fic as true and factual events (its why true story film say "based on" or "inspired by" actual events rather saying these are true and actusll events). RPF though, as far as copyright goes, almost definitely entirely protectable.
Tl:dr you do own lots of elements of your fanworks, just not the specific bits that are pulled wholesale from canon. RPF surprisingly even more protectable.
