Once a work is published it becomes a source. Just like the real world evolves, worlds created and introduced in fiction are also subject to evolution and readers use the source as reference material to further develop the world and its inhabitants. This is fanfiction. Fans just do the work for fun. I imagine that this can be scary for an author who sees their text as their brainchild and is protective of its world and inhabitants. But for readers, the text serves as scaffolding and incites creativity. Publishing a story is unleashing it into the wild and there is no real way to determine or control the outcome(s). I’m not sure one should want to. Authorial intent often seems like damage control unless there is an author’s note that explicitly discloses her intent when the work is initially published. For an author to respond rather than initiate the conversation seems to undermine the value of literature. Texts are shaped by authors but that shape is not necessarily fixed.
Unlike fanfiction, adaptation is professional work. There is an obvious reference to the original work while it is translated into a new medium.