Just chiming in with a few ideas:
1) The future combatants are not specified, so why not humans AND parshendi United against Odium? That would help strengthen the justification of the treaty while trying to bring about war.
2) The last book (not counting the novella) ended with the introduction of nightblood, who talks in people's minds, into storm light. The prologue mentions a voice in someone's mind. Coincidence? We've seen nightblood in the hands of minor characters before and what did it do? Try to get back into the hands of a major character. Who better than Szeth to slay evil?
3) GAvilar was working with a herald, but which? We have three candidates for potential heralds at the meeting one basically random dude and two random chicks. I like elhokar 's wife, who we already know Kasnah suspects of something, for one of the chicks. Or maybe Dalinar 's former wife; she's super sketch. Probably not both. Are there any princes who we've seen wear old school uniforms? If so, that'll be the current identity of our rogue Herald.
4) why are all those Heralds there if the Gavilar brigade is trying to force then out of hiding? Was Gavilar being double crossed by his own (herald) informant?