And then the fun and games begin.
It's easiest when the thought is that the writer really made totally inadequate use of that idea, but even there it can be problems prying the gems from the gimcrack setting they were in. And when your thought is how cool, cool, cool that idea is, I want to play with it, the problem may arise that all of it seems charming. It's hard to tell which of it is the idea that fascinates and which is the charming but unnecessary setting.
Sometimes trying to mix it up with another idea helps. Then, sometimes it brings up the magnetic attraction of the whole story. Like, thinking the heroine in one notion has blond hair, and in the other, brown. Which is silly. After changing the name, changing the characters' physical appearance (particularly if lifted from a visual medium) is exactly the sort of superficial change that allows you think of them as independent of the source.
But sometimes it takes several shies at it to persuade the notions to do that sort of thing.