This one was easy for me. I love working with theme, and that's often one of the first things, after character, that comes to me. It's the touchstone I use throughout the writing process, and it's just naturally the way I think about what I write.
I didn't already have a theme for Isaiah, but it didn't take more than a few hours of brainstorming to come up with one I like, and that I think works with the plot elements, tone and characters I already have. Which is good - I really don't want this to actually take the whole two years!
So, my theme, in one word, is Faith.
Expanded: Faith is the gateway to the extraordinary.
Edit: Before anyone thinks I'm writing a scary religious book, by "faith" I don't mean religion but the basis of religion - a combination of belief and trust. You can have faith in anything.