My longest, most popular fanfic ever, literally started off as nothing more than: "What if this character started off as a prostitute instead of just a cab driver?" I had NO idea where I wanted to go with the story other than that, but I just started writing it and kept on with it. After maybe 6 or 7 chapters, I finally realized, "Oh crap, I really need to figure out where I want this to go." Amazingly, a surprisingly coherent story managed to evolve from that simple idea and I was able to put together a full outline for the whole story arch. So my advice would be, just start writing, and see where the writing takes you.