I'm working on a fantasy novel now that germinated from a mental image I had nearly ten years ago, of a man walking through a field of tall grass. That was all I had for a long time -- a young man in medieval looking clothing, walking. Where he was going and why took a long time to become clear. (Hopefully someday I'll finish that one!)
Often, my initial idea for a story is a visual scene like that.
If I'm very lucky, it's a scene that comes into my mind of people talking -- like a couple of characters I am overhearing in a restaurant, and then I can fill the lives around and extending out from that conversation.
Some writers start with idea or theme. I can't think of a single time that's happened to me. Usually my ideas or story buds are characters, or visuals. If I'm really lucky, if I'm really in the zone, I can just transcribe the movie playing in my head, and the writing seems effortless.
If I'm not so lucky, I have to listen really hard, use music to set a mood, or return to movies and TV shows and books and paintings for indirect inspiration.
But probably because I love romance best, and romance is a very character-driven genre, I see and hear the characters first -- people, in a certain setting, doing something. That's where my imagination seems to be happiest -- in the visual.
Theme, moral, tone -- all that brings up the rear for me. Even sometimes not coming clear until the third or fourth edit! But I've learned not to fight the process. I simply go with what works.
Where do your ideas take shape?