I also have no idea what to write about - original fiction vs. Tolkien fanfic. The character I have in mind won't fit into Tolkien's world, whichever time I choose. Unless I deal with Easterlings' or Dunlendings' past, and that is equivalent to writing original fiction. Maybe late Numenor but even there it wouldn't really work. She just doesn't fit into ethical and metaphysical boundaries of Tolkien's world.
I have a character in mind, I have to create a world where she could live comfortably. It would have to be very similar to this one. Because, essentially, it would be a modified self-insert. And she will be a villain. I know her personality and manner of smile, her likes and dislikes, but I have no idea what her name is. What kind of people she would come from. What kind of language would she speak, what she would look like, what her childhood was like and her death.
I just know that what inspired me most was Limyaael's rants, especially rants on villains. I always feel so sorry for pathetic villains of Hollywood movies.
Or maybe what I am really itching to write is a discussion on physiology and biochemistry of [Tolkien's] Elves...