Okay first I'd like to say this probably should be called a Hypothesis rather then a Theory, but I don't think many here will be overly upset at the title. I have very little evidence (if any, but you judge for yourself) that supports this, and truthfully I fully expect to be proven wrong as we learn more about Sanderson's Shadesmar/Cosmere Metaseries, as I like to call it. But this thought has been bouncing around in my head for the past couple of days and I'd really like to see what others think of it.
So lets start with a quote.
In response to Kaladin asking Hoid's name, he says:
All of that is very interesting, as is what he says later in response to Kaladin's continued questioning, but what I want to talk about is what I put in bold.
When Hoid said he began life as a thought, the first thing I immediately thought of was the Cognitive Realm and how the Shards had to acquire humans to be their minds. We see this with Ati, Leras, and Vin for sure in the Mistborn series, and the Way of Kings provides names for many others.
Its pretty clear from Mistborn, Warbreaker, and now the Way of Kings that the Shards of Adonalsium are incredibly powerful, often treated as Gods by the people of their respective planets. And if they are in fact pieces of something greater, as is implied in the fact that they are called Shards of Adonalsium, then it should be fairly obvious that Adonalsium was something even more powerful, something closer to a God in the traditional sense I think. Something with a mind.
But as we've seen, the Shards need people to act as their minds. Which begs the question where did the Cognitive aspect of Adonalsium go? Perhaps as a result of the Shattering its mind was separated as well. Perhaps that mind eventually went to become the entity we know as Hoid.
Now aside from that one line, and the odd fact that the Shards of Adonalsium need people to be their Cognitive aspect, I have no real evidence for this. In fact there is no evidence that Adonalsium even had a mind, it might well have been an object of great power. I don't believe this, but I figured it might be good to point out the obvious flaws in the random idea that popped into my head.
EDIT: I've actually come across some evidence that makes this idea less likely. If Hoid is in fact the author of the epigraphs then it would be very strange for the Cognitive aspect of Adonalsium to have known Rayse on a personal level to the point where he has a grudge against him as the epigraphs suggest he does, since Rayse was presumably a human being the same as Vin, Leras, and Ati were.