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That depends on the definition of "content". Both copyright and author's rights law protect the specific expression of an idea: if you draw Superman yourself, from scratch, without e.g. tracing original art owned by DC Comics, then no, it's not so clear at all. It depends on the owner, jurisdiction, and whether you're making money with it, but you won't automatically get in trouble.
It is not a gray line, what happens is that many companies tolerate it, but if they wanted to, they can remove your fan-game and legally they have the right and there is nothing you can do to defend your work.
If it really was a gray line, you could go to court and expect to win and that doesn't happen.
Companies like "Harmony Gold" every time a fan project becomes notorious, they knock it down.
Companies like Capcom have unofficially said that they have no problem with fan-created content, as long as it is respectful.
Companies like Sega have closed projects, etc.
In the case of itch. The rules say that you must have the rights or permissions to all content you upload, but this is not enforced.
You can upload a fan game, but if the owner of the content files a DCMA, Itch will remove the game, don't hesitate.
Maybe you are confusing "fair play", where it is legal to use content that you do not own or do not have authorization for, but a fan-game does not fall into that definition.
Edit::
Examples:
https://www.wired.com/story/sega-shuts-down-streets-of-rage-remake/
https://www.gameinformer.com/b/news/archive/2010/03/02/fan-made-king-s-quest-seq..
The thing is that not only the sprites or music are protected, but also the names and characters, for example, Superman.
If you draw Superman from scratch and place that drawing in your game, it is still Superman, which is copyrighted content, it doesn't matter if you drew it from scratch. Drawing it from 0, by you, does not give you the legal right to sell or distribute said content.
As @Eris Lord Freedom says, it would have to be a "Transformative" work, for example, a superhero who flies, but who is not called Superman and who is not so similar.
When you make a fan game, it usually refers to names and content that is the property of a company, even if you create the sprites or music.
You can create a VN about a mustachioed plumber and a princess in a fictional world inhabited by turtles and you'll legally be safe.
But can it be considered a fan.game?
The moment you make a VN of Mario and Peach, what would be a fan-game. You also have a copyright problem, because that Mario and that princess are protected content.
As you say, there are certain lines that are not clear and could only be resolved in court. But in this case we are talking about Itch. If you make a VN with Mario and Peach and Nintendo comes with a DMCA to Itch.
What do you think will happen?
It is based on that that I always consider and recommend considering that any fan-game can be removed from Itch if the rights holder wants.
PD:
For example this case of harmony gold against argentine developments (sorry, only found in spanish)
https://www.guioteca.com/anime/harmony-gold-censura-produccion-de-fanaticos-arge...
This person had created some videos that told a fan-fiction story of an anime called "Robotech" all the content created from 0 and still had to delete everything.
He could have changed the name and designs, in such a way as not to use protected content, the problem is that by doing that, the product cannot be directly related to "Robotech" and therefore it would have ceased to be a fan-fiction of "Rototech".
Copyright 'law' is just a tool for rich people to maintain control over their cash-cow product. If the rich person doesn't COMPLAIN... poor people can do whatever they want. If the rich do complain, the poor lose. period. Know what the rich will tolerate, from previous examples, and don't cross the line.
Copyright law is also how poor people avoid being exploited by mega-corporations.
It's not easy for an individual artist or self-published writer to sue Disney - but copyright law is what prevents Disney, Paramount, Warner Bros. etc. from just mass-grabbing stories and art and turning them into movies for their own profit. The fact that the law exists keeps them from grabbing everything they can.
There are occasional cases of "we're pretty sure this was grabbed from someone else's idea," but Sega couldn't grab Flappy Bird and distribute it through their channels because of copyright.
Copyright law has problems and needs a major overhaul but it is better than no protection at all.
I like your reply because it is true... but in all fairness, the creators of 'flappy-bird' (or any successful game) aren't poor anymore... so technically, copyright law is protecting the rich... because they are now rich. In reality, the VAST majority of TALENTED working artists are "poor" relatively-speaking, and (from one point of view) are being "exploited" by the rich... It's just called "employment", where the poor do all the work, and the rich take all the profits and own everything the poor create. Yes, that's just how the world works... but, it still ends with: Copyright 'law' is just a tool for rich people to maintain control over their cash-cow product. (99.99% of the time)
Maybe this answers your question
https://itch.io/docs/legal/terms#4-publisher-content
Publishers are solely responsible for the content they upload and distribute on itch.io. Publishers affirm, represent, and warrant that they own or have the rights, licenses, permissions and consents necessary to publish, duplicate, and distribute the submitted content. By submitting content to the Service for distribution, Publishers also grant a license to the Company for all patent, trademark, trade secret, copyright or other proprietary rights in and to the Content for publication on the Service, pursuant to this Agreement. The Company does not endorse copyright infringing activities or other intellectual property infringing activities and violations of may result in the removal of content if the Company is notified of such violations. Removal and termination of accounts may occur without prior notice.
Fanworks are a legally gray area because some use of other people's works without permission is allowed. This includes using pieces for reviews and commentary, and transformative uses of the original work. The trouble is figuring out what's "transformative."
"Derivative" works are a copyright violation (in the US) IF the copyright owner complains. But if they don't care, it's fine, and they can decide what's worth their effort; it's not like trademark where they have to do something about infringing works. They can decide "that fangame is derivative, but I don't mind it; I'm not going to have it removed" and they can still go after other copyright violations.
"Transformative" works, including "parody" (which is a legal term; doesn't mean it has to be funny), are legal uses of someone else's material, even without permission, even if they hate it.
The line between "derivative" and "transformative" is decided in court, for individual cases, and is inconsistent as hell. People write dissertations on this. People get PhDs trying to describe the differences.
But the key part is: Only the rights-owner can complain about copyright infringement. If it's not your IP, it's not your problem if someone else made a game from it.
If you want to make fan games, your best way to make sure you're in the clear, is knowing that the IP holder doesn't mind them. (Actually best is formal written permission, but nobody is expecting that. But knowing that the original rights holder is cool with fan games is pretty close.)
Japan has a much different culture about derivative/transformative uses of copyrighted material, which is why you see a lot of anime/manga zines and doujinshi and fangames.
(IANAL; TINLA)
If you don't want to worry about copyright issues, then yes, use only material you have made. But that's not what's required. People are allowed to use some aspects of other people's copyrighted material legally. How much, and in what way... that gets complicated.
A lot of us want to build in what other people have made - want to show it in a new way, or have a different perspective on it, or use existing characters with different game mechanics.
I tend to think this is transformative use, not derivative - but I am not a video game developer and not a lawyer; it's not my opinion that matters.
I can say that the most common reaction to a big company deciding something is derivative/infringing, is to demand it get removed; there is almost never any other penalty. (Going to court takes money. Even if they would win the court case - there's no point in suing a small indie developer; they won't get that money back because most indie devs don't have it to give.)
You're also not likely to put yourself in trouble. Normally, they send you a cease and desist letter and you have to stop distributing the game. It is almost impossible to get sued or go to court.
As they recommend, look for information about the company that owns the rights and see what they think about the projects created by Fan.
For example, Harmony Gold is a very aggressive company and a project that finds, a project that closes.
At the other extreme, you have Capcom, which tends to be very permissive with fan projects.
Well said.
Also many people confuse copyright with trademarks and vice versa. And some "fan" works do violate copyright. Or trademarks. And in some cases even patents, where the broken law allowed patents on software.
And both copyright and trademarks do have their fair use cases, wich means that there are situations where the right holder can do nothing legally to stop you from doing it. Oh, they can do some non legal things that appear legal. Like frivolously suing you and hoping you are in a situation where you will not go to court. I did mention the broken law systems, did I.
As a rule of thumb, there should be no way in hell anyone can be confused about the creation as coming from the trademark owner. It also helps, if you are not competing in the same market. Those laws are made to protect. If you create brand x and someone makes a cheap knock off brand called k cashing in on similarity, brand owner x can sue copy cat brand k.
Now to put out a hypothetical scenario. You make a parody game of Superman doing homework. Since I do not think that there are any such games out there by the IP owner, I guess there will not be any copyright violations possible, as there is nothing to copy from. Should you trace some comics however, or use screencaptures, you are in fair use territory of copyright and have to figure out, if you are still doing fair use or not. And of course, is there trademark issues ? Is DC in the market of indie parody games? Will the game hurt sales of their own games? Does it damage the brand? Typically I would say no to all three and even claim the contrary for all three. It is free advertisement for them. It increases brand recognition. No one will buy a fan game instead of the real AAA title. This is not some motor oil or softdrink. But, alas, this is legal territory and there is a reason why lawyers and big companies have a bad reputation. So I am stealing that one: (IANAL; TINLA)
Still, I think it is an explanation why there are so many "fan" games out there not taken down. Not only because they do not care, but because in many cases, because the game actually does not violate either copyright, nor trademark.
The risk is that your game could be taken down. Online marketplaces don't necessarily have the resources to police everyone's content... they say not to use content without permission in their rules so that they don't get in trouble for hosting your rip. The larger stores might screen your game, but in any case the main concern is that if your infringing project gets enough attention, the original copyright owner will be the one coming after you with buckets of cash and a vested interest.