I'm going to talk a bit about fanfic. And about 'feature-based' RPGs or RPGs based in somebody's world.
Some authors hate fanfic. Some are terrified that a fan writer will sue them if something too similar to a 'published' piece of fanfic shows up in the original authors' work, resulting in authors demanding nobody ever write fanfic or, in at least one case, game in their world or, more commonly, in authors publicly declaring that they will not read any fanfic set in their world or using their characters. They can then use 'I never read it' as a defense.
I admit. I do the RPG thing. I don't write and post fanfic...I have enough ideas of my own not to have to base things off of other people's, but I am a gamer. And sometimes I game in published worlds.
Over ten years ago I was involved in an online RPG and created a character who was a close relative to a comic book character.
Today I opened GLC #0. There, on page 3, was my RPG character in a cop uniform. Same appearance. Same name. Different relationship to the original character and different role...but those differences were timeline differences. MY Gloria Gardner was Guy Gardner's daughter and successor. Theirs is his cop kid sister...but it was recognizable immediately as the same character.
Am I going to go after DC for stealing my character? Heck no. I started this blog post when I finished laughing about it.
The chances of anyone at DC knowing about my version of the character are slim. And obviously Guy's daughter and his sister are likely to look a lot alike. The name 'Gloria' is an obscure in-joke only long-term fans will get, referring to an early obsession Guy had with the Silver Age character General Glory...so the same name? Not likely a coincidence.
And, above all, these things happen. It's frustrating if somebody publishes a comic with a character that has the same costume, codename and powers as one you were about to make, as happened to somebody I know.
But I certainly have no moral claim to Gloria even if I *did* come up with her first. Morally, a fanfic writer or gamer has *no claim* to what they create using somebody else's world. Now, you can file the serial numbers off and publish the story as original...and even have a lot of success. (Fifty Shades of Grey started out as fanfic). Being inspired by other people is no crime.
Going after an IP owner because they 'stole your idea' that you made using their property...that's the kind of thing that gets people to decide they won't sell the RPG rights to their books and will send nasty legal letters to teenagers. If you're playing in somebody else's sandbox, have the respect to acknowledge that it is their sandbox and your work would not exist in its form without that.
And accept that sometimes two people working off of the same basis might come up with the same idea completely independently. There's a reason you can't copyright *ideas*.