So Harmony is made up of two opposing Shards. As of MB era 2 Harmony is still mostly Sazed, as far as I understand, but over the years his shard's Intent is going to remake him into... something. I see four options as to what that something might be:
1. Ruin and Preservation Intents together pretty much cover all potential actions Harmony might take, so they cancel each other out. Future!Harmony stays Sazed inside and can do whatever he likes. (Pros: Adonalsium was all those conflicting proto-Intents at once. Cons: seems like too much of a narrative free lunch).
2. Pretty much all potential actions fall under either "conflicts with Ruin's Intent" or "conflicts with Preservation's Intent". Harmony ends up not really being able to do anything. He spends future eras basically sitting around on his thumbs. (Unlikely; sounds like a waste of the character).
3. Harmony can only take individual actions that are equal-parts Ruinous and... what's the word? Preservative? Preservation-y? Not technically as limiting as #2 (but still feels like a waste).
4. Harmony's actions have to average out being Ruinous and Preservation-y. So if he wants to, say, save a major city from a tornado, he can do so, but he'll have to cause an equal amount of destruction (either at once or cumulatively) somehow somewhere on Scadrial. (I find this one the most likely, if only because it seems like the scenario Sanderson can do the most with).
What do you think?
(sorry if I've used the wrong words or spelling; I don't have time to edit this properly)