A theory lodged into my head about the main antagonist of book 6-10.
I think, Cultivation is going to merge with the Shard of Odium. This is why:
During the Navani Raboniel brainstorming sessions, there was a glaring omission. Neither of them suggested, or even had a passing thought about mixing voidlight with lifelight. Both of them were scholars of wide branching interests, they had the method, they had the materials, they even had the free time. Raboniel learned a completely new rhythm from the storm-void mixture, so why not find out how the life-void mixture sounded like?
I couldn't think of a single excuse to this, unless the author himself wanted to avoid attention, because naming the rhythm of the light would give away the antagonist of a future book.
So what do we get if we mix the two? A tone of chaos mixed with the tone of ever-building majesty? A tone, which makes chaos build something majestic? It sounds a lot like the Rhythm of Evolution.
The chaotic shuffling of genes, and the passionate struggle for survival mixed with the uncaring selective pressure of nature. Nature, which is slowly changing, thus forcing everything around it to endlessly adapt and improve.
And to make it more interesting, add to this the intelligent guidance of selective breeding, and the power to grow whatever you want.
This works SO well. Cultivation has been established as an entity lacking of passion. She doesn't care what she grows, she only cares about growth itself. For her a boon and a curse are equally valid growths. But add passion to the mix and suddenly she can have directions and great goals.
I suspect, that the Shard Cultivation literally cannot have passion for her creations. Its Vessel Koravari might want to have it, but the Shard resists. We learned in RoW that Shards and their Vessels can have opposing desires, and usually the Shards win. If Koravari took Odium, she could finally grow things with unbridled passion.
But to what end? What would this mean for the Cosmere at large? Well, there is the utterly terrifying image of a rapidly evolving godlike zerg species taking over worlds, but I think that is just the means to an end. So what is the end goal for the God of Evolution?
I think she wants to grow, to evolve back into Adonalsium itself. She wants to merge all the shards, and probably try to recreate the dead shards too. This might be possible if we look at it through evolutionary principles. Without getting too technical, for example if an apex predator dies, that opens up a niche, a spot where another species could evolve into, thus taking its previous role. A similar yet unknown principle could exist in the Cosmere. Maybe the Shards that have died left their imprints, which divine evolution could fill up.
Who could better accomplish this than a god whose very nature is to change and adapt? To build itself to become ever greater?
This seems like the sort of villain Sanderson would write for his endgame. Odium is kind of a bog standard world conquering maniac who wants to become the sole god of everything. Not nearly interesting enough to keep until book 10.
There are other clues to Cultivation being up to something big:
Book 6 will be focused on Lift. This is important, because Cultivation has only ever touched three mortals. With Dalinar, she stole Odium's champion. With Taravangian, she stole his vessel. But we still have Lift. She has to have an equally grand role in the coming events, but what else is left to be stolen, other than the very Shard itself?
The powers that Cultivation gave her are quite telling too. She can enter and manipulate the visions of gods, the same place where Rayse just got killed. She also uses the light of cultivation exclusively, and has some hidden property to not get changed (Resist the influence of a Shard maybe, but able to seed it with lifelight? We have to wait for future books to learn.) Point is, she reeks of Cultivation, and if we keep with the pattern, this will somehow oppose Odium.
Finally, even though Odium is confined to Roshar right now, what would happen if he merged with Cultivation? Is it possible that all previous agreements would be null and void, since Evolution, as an entirely new Shard, has never agreed to anything?