Where do the ideas come from? From nowhere.
I thought I'd write out a scene between Fiona and Graham, one I had a vague idea of how it started. Then an idea appeared. And another. In no time I had over 1000 words, some great ideas - taking the piss out of Dan Brown, adding some conceptual continuity, some great dialogue exchanges...it was wonderful. In one sitting I was able to write some great words AND deal with some of the plot. And where did it all come from? Nowhere, that's where. The process of writing created the writing. It's true what they say, sit down and write. Something will appear :-)