So, I recently read the Hope of Elantris, a short story available on Brandon's site. And since I'm not required to place spoiler tags here, I'll just run with it.
So, we know Ati is the man Atium was named for, and holder of the shard Ruin. In Elantris, and the Hope of Elantris, Aon Ati is 'Hope'. And from the Letter, we learn that Ati 'was once a kind and generous man'. So, it could be a coincidence, the way things are going down, and Ati and the Aon share a name. But, I've noticed with Sanderson, there are few coincidences. So, to the meat of this idea: I think that, like Hoid, Ati was a worldhopper (a member of the 17th Shard, perhaps?) and thereby found Ruin.
It's a solid theory, I think... However, knowing that I don't know everything, I'm interested what everybody else has to say.