Question

Like copyrighted works, or a prompt/idea? I'll see if I can clear a bit of this up for you, but please correct me if I'm misunderstanding your request.

Most "fan fiction" (i.e. fan-created derivative works of major trademarked characters and entities) are covered under the umbrella of "fair use" as parody/artistic interpretation/etc. So long as the created media isn't making money off of the original intellectual property, you are generally going to be just fine. So, for example, if I wanted to make a story about, let's say Sonic and his friends, I am pretty much golden- so long as I don't profit from the work, nor represent the work as official.

Now, going deeper, we have Original Characters, or OC's. This website is full of them. This will vary from artist to artist, and you must without exception ask permission to use another artists characters. It's just good form, and 9 times out of 10 they are going to let you use their characters/settings/etc. If they decline for any reason, you're wise to respect that, or risk not only that artists' wrath, but the wrath of other artists for "stealing" other works. It's a largely unspoken code, but it's easy to follow- if you don't know, ask before you use something that belongs to somebody else, and respect their answer. It's just good practice.

Finally, prompts. As I see you're a writer of sorts, we tend to rely on a "setup" rather than say a particular character or place to get the ball rolling. Usually it boils down to "X does Y that then leads to Z" to put it simply. These are more nebulous, as I don't know that anyone can claim total dominion over an overarching concept, and the notion that with a few minor tweaks you can take one prompt and make potentially hundreds of unique works based on it. To be safe, when I'm about to cook up a story, I ask myself if I've ever seen it done similarly before. If it's a real close comparison to someone else' work, I either modify mine to make certain the two works are incomparable, or ask the creator's permission if I think that my story might encroach on their ideas too much.

Full disclaimer, I am a car salesman by trade, not a lawyer, so I cannot guarantee this is all totally accurate nor up-to-date.