The Returned from Warbreaker are talked about a surprisingly small amount on the forums, and the workings of magic on Nalthis are amongst the subjects we know the least about. Even the characters from Warbreaker know very little about what it is they do. As a forward, this theory relies heavily on the assumption that The Principle of Intent by Chaos is correct. If you have not read it or do not want to take the time, I will give you the main point:
What this means is that the magic systems of a world act accordingly with the intent of Shard they are associated with. Preservation preserves, Ruin Ruins, Endowment endows, etc...
About midway through the theory, Chaos diverges into a side of speculation
This Immediately struck me as having importance and being worthy of investigation. It also brought to mind a question that has been nagging at the back of my mind since I read Warbreaker: Why do the Returned need additional breaths to live?
If magics act according to the intent of their shards, and the returned are given their Divine Breath by Endowment, it does not make sense for them to require additional breaths to live. It does make sense, however, if the returned were never supposed to live forever in the first place. Let me explain. Taking the Breaths of others is an extremely Un-Endowing action, it is essentially the opposite of the intent of Endowment. If it is true that Ruin and Preservation could fuel all of the Metallic Arts, even those not in accordance with their intent, but doing so would require them to expel their power without it returning, then it is also possible that the returned could go against Endowment's Intent but would require the expelling of energy, or a source of NEW energy to do so. This is where the required breaths come into the picture.
Brandon has said (citation needed) that the focus of BioChroma is commands, not the breath. Breaths require a command in order to awaken an object. Why then, would a Divine Breath, essentially one breath with the power of many, be exempt from this principle? It would not be.
My theory, which I have dubbed The Endowment Axiom, is that Divine Breaths are (or were) given commands by Endowment to awaken the Returned in order for them to endow their power on one improvement for the world.
Why is this important? We already knew the gist of this before. This is important because if the Returned continue to live without using their Divine Breath to aid others, they are acting against the command, against the intent, of Endowment's Shard. Acting against Endowment means that, at the end of their determined lifespan (I believe a week but need citation) their power (the Divine Breath) would dissipate and they would die. They are able to survive longer than intended by being infused with more of Endowments energy. This energy fuels the life of the returned even when they should have died.
The largest possible criticism with this theory (besides it's reliance on other theories) is that if the Returned living goes against Endowments intent, and magic against the intent of a shard requires energy to be permanently spent, Endowment's energy should be diminishing or eventually run out entirely. I do not believe this to be the case, however. The returned are not running out of energy, but receiving new transfusions of energy every cycle. This new transfusion allows them to continue living without permanently expending their energy. Another criticism of this theory could be that the Returned would have to follow the command given by Endowment. This has, IMO, two possible solutions. The first is that the Divine Breaths only have the command to awaken the returned, and the intent of the Shard is supposed to press them into giving it up. The other is that all humans have part of Endowment in them already in the form of their personal breath. This spark of Endowment could give humans free will and the ability to resist giving up their Divine Breath.
Implications this would have if true: Endowment designed the Returned to be brought back and use their breaths relatively quickly, Divine Breaths have/require commands in order to function. This would not mean that normal people are supposed to give up their breaths and become drabs, because the Divine Breaths were given the explicit command or intent of being given up, whereas a persons personal breath was not.
I think we will eventually be able to look at every shard and see their intent's influence on their respective magic systems. I put this in general theories because, while it mainly deals with Warbreaker, it has some spoilery details of Mistborn and references other general theories about the Cosmere.
Edit* Edited the topic description