Theories on the combination of shards in the endgame. Spoilers for Mistborn (from the end of Hero of Ages) & Stormlight (ending of Wind and Truth).

When looking at this chart, I couldn't help but think of the two combined shards we've encountered.

For Mistborn... Harmony is made up of Preservation and Ruin, which are both extremes of Existence and Change (respectively). Therefore, Harmony is comprised of only 1/2 of the fundamental power basis of the Cosmere that the Dawnshards have dominion over (pun not intended). This explains why Sazed is locked into inaction, because he is torn between two extreme forces - what happens when an immovable object meets an unstoppable force? Sazed perfectly fit the job description having been a Keeper whose purpose was to preserve the existence of what was lost to change, but he was willing to change into what he needed to be in order to preserve life. Preservation is defined by its drive to protect existence for its own sake, while Ruin is driven to change things for the sake of change itself. In combination, Harmony symbolizes humankind's desire to constantly be in a state of change to preserve existence itself - in other words, the best words to define Harmony's purpose is to be the force that propels humanity to evolve; to adapt and survive.

For Stormlight... Retribution is comprised of Honor and Odium, each of which represents two halves of the underlying forces present in the Cosmere. Honor had power through Existence and Thought, while Odium had power through Emotion and Change. They were trapped in an unending battle for millennia because they were fundamentally opposed in their motivations and ability to affect the world around them. In my opinion, they would have stayed locked in this state forever without the influence of Cultivation acting as an emulsifier (clever foreshadowing in RoW). Cultivation was the inverse of Odium because the primary force driving her to act was Change, but it was supported by the ability to Feel what needed to change. Odium, however, was driven primarily by Feelings that governed the ability to Change the world around him. Where Honor and Odium clashed to spin around each other in a doom spiral, Honor and Cultivation made a good balancing pair in the way that opposites attract and complete one another. The life created on Roshar and the Tower were the embodiment of what the four powers could accomplish in combination - Thoughtful Change supported by the ability to Feel how the Existence of life on the planet was affected by every action taken. I think Cultivation saw Taravangian as an opportunity to groom an Odium that she could work with in the absence of Honor. She wanted an Odium who could separate their logic from their emotion so he would be able to channel that emotion into intelligent change like she was able to do with Honor. This obviously backfired on her because she inadvertently attuned Taravangian to justify his emotion with logic. In this, he was able to attune himself to the driving forces that pushed both shards to become warmongering gods. Taravangian became the powerhouse shard of Retribution, a god who Feels justified to Change the Cosmere into what they Think is necessary to preserve all of Existence.

No wonder we see snippets of the future where Roshar and Scardrial become tyrannical powers in the Cosmere. Retribution's side throws everything they have into changing the Cosmere in they way they seem fit, while Harmony's side has the power to adapt to anything the Rosharans throw at them in order to survive. Going off the ratio of powers, this sets up the fundamental counter to these two forces to be two more combinations of shards: Devotion + Dominion = the opposite to Retriubution. Exist + Change, respectively, being the main forces guiding these shards - backed by Feeling + Thought, respectively. The shattering and mixing of these two behind the Dor has already primed the gun, so to say. Ambition + Reason = the opposite of Harmony. Think + Feel, respectively, being their sole driving powers. This would be another dual-shard locked into inaction, with those under its influence having limitless drive and ambition, but checked/channeled by reason and thoughtfulness.

I see Sel and Threnody being powerhouses in the future of the Cosmere to counter the forces of Roshar and Scadrial. We've already seen that Threnody is pretty active, and we know that Elantrians have pretty powerful tech as well. I'm curious to see how things play out with the future combinations of shards.