The trick, then, is to make an engaging and fresh genre piece. Which is, frankly, a tall enough order, thank you very much.
So that said, a few words on theme.
The world is so frightening. It's so big, and it's forces are so vast compared to its inhabitants, and everyone with a shred of knowledge or power spends most of their time trying urgently to tell you that everyone else with knowledge or power is going to - very quickly - end the world. It can be hard not to believe them.
I re-read 1984 recently. It terrified me, more than when I read it as a teenager. I don't (necessarily) mean to say I think the world is falling apart. Maybe it is, maybe it isn't. The point is, we take so much on trust, and it's not really in anyone's best interest to be honest with us; it's in their best interest to persuade us. There are honest voices, but they're not necessarily the most trusted or powerful, and it can be very hard to tell them from all the others. Given that, we all believe many lies to be true. It's unavoidable.
Thematically, that's what I'd like to treat: the idea that truth is complicated, deception is easy, and institutions and individuals have incentives to lie.
What leapt to mind, for a fantasy setting, is the concept of prophecy. Prophecy is, of course, a rather tired bugbear. Prophecies are frustrating, aren't they? They kill tension, so in order to get it back, any story that includes one becomes a game of, "How different from what the prophecy clearly states can things be, while still technically fulfilling the terms of the prophecy?" Once the prophecy is made, there are basically two possibilities:
- The consequences are averted through tricky wording: "Yes, the Slayer died fighting the Master, but the prophecy didn't say she'd stay dead!"
- The consequences are felt, but at an unexpected time or in an unexpected way: "Yes, Oedipus married his mother - because Jocasta was his mother all along!" Well, I'm sure it was unexpected at some point.]
I would be remiss not to acknowledge the fine work of the inimitable Fred in discussing the bald-faced prophecy-related lies in the Left Behind books. His nuanced discussion of honesty as it relates to holy scripture is no doubt incredibly influential in how I'm thinking about this.