Sazed's Shards
I was absolutely convinced Sazed would be Discord by the end of The Lost Metal and I'm a little thrown that he wasn't. My next theory is that its just set that up for era 3.
It seemed so obvious! His indecisiveness was causing problems, and if he'd been able to shake off some of the constraints then he could have beaten Autonomy himself, surely. He holds two Shards.
He couldn't act because he was Harmony. Not to help people and not to fight Autonomy and it was frustrating everyone. Even worrying Sazed himself.
I think he'll be Discord soon. And it'll happen in a positive way. I think the Shards of Ruin and Preservation clashing against each other might actually help Scadrial, because Sazed will be more active against outside threats which are clearly a big issue.
(It might even also hold his independence as a person a little because maybe if the Shards are less balanced they'll overwhelm him less. I really think that might be important. The power of the Shard seems to always overwhelm the bearer and I haven't seen good results yet. This is a reach but I like Sazed! I can hope.)
My concrete evidence for this is that quote from the Hero of Ages prophecy. I dug it up again, to make sure I was remembering right:
" His name shall be Discord, yet they shall love him for it."
Discord will happen, I'm sure of it. But it'll be a good thing.