I can't claim to have ever ripped off a game for anything I've written, although I will mention one.
I was reading through the Eberron Player's Guide for D&D 4E, and checking out the Changeling race. It occurred to me that the Changelings' ability to alter their appearance instantly and permanently, with apparently little or no effort expended, is far, far more powerful than any of the other races' powers.
While I was out raking thatch in the yard, I started thinking about the ramifications of that power, and how it could completely change a campaign. Then I starting thinking about how it would completely change the world. And then, most importantly, how it would change our world.
So I sat down and planned out eight different stories, each centering on a different person, each a changeling in the modern world. Six were positioned somewhere along a cynicism-idealism scale, with the other two used to address a scientific and philosophical/religious viewpoint, respectively, about changelings' interactions in the world and its culture and events.
A week and 40 pages later, I had Changeling Tales. It's by a wide margin my favorite work, and probably my best. I tried to write each chapter with a unique, character-centric style, and to make each character feel fresh and interesting to serve the end of providing a good cross-section of changeling society and to keep the narrative fresh and varied.
I also tried a couple of very novel techniques with the format of the text itself, both of which are, in hindsight, interesting but very troublesome. The text has to be kept in its original format or it becomes impossible to read. And even if it is in that original format, it is still very, very hard to make sense of. I'm very glad I tried it, but it was a lot of work for a very flawed outcome, and I wouldn't do either of them again. I realize this paragraph doesn't make a lot of sense.
There are a few little things I'd change about it, looking back, but that's a bad habit to develop. I'm happy with it.