Actually... I think we got our ideas crossed there. My idea with the Lord Ruler was that, in fact, the Lord Ruler did leave clues behind.
My thought is that the Rashek, at first burning the power, had no idea what he was getting into. He changed things unintentionally, without knowing about the dire, dire consequences. However, notice the difference in tone between the MB3 epigraphs and the Lord Ruler in MB1. There's almost a hesitation in him, directly post-Ascension.
Perhaps his mind had broken into what he turned into later, consumed from the guilt from what he'd done, transforming him into the hard, cold Lord Ruler. However... before that...
During holding the power, he thought he destroyed the Deepness, but he only managed to push it back. No, he burned the power away before he could realize how to defeat Ruin truly. Holding the power could, perhaps, given him the knowledge to do what he needed to do, but he didn't do it properly. It was gone before he knew it. But later--perhaps--maybe he realized the true way to defeat it. To this extent, he placed the Well under Kredik Shaw, so that when the Well came back into power, he could ... finish the fight.
But before that, he probably would have left clues, maybe for his own use (if his metalminds couldn't be trusted). In fact, at Statlin City, there is one of two possibilities: A Clue, or one of these storehouses Elend has been raiding (or both). Notice that I distinctly exclude the possibility of the atium stash being there. This is because, narratively, it wouldn't work. There's a year between MB2 and 3. I'm sure Elend or Vin would've investigated Statlin City in that amount of time. Thus, if they investigated it and found what was there, it would all be off-screen. The atium stash is too big of a plot point to happen off-screen. Thus, it's more likely it's a clue. Or a storehouse. Or both.
I need to elaborate on the Vin knowledge thing. Since reading MB2, I've always wondered how our heroes would learn of Hemalurgy. The Lord Ruler isn't alive to tell it, and Marsh would be more likely to kill them than have a nice conversation about the basis of his power. Now, in our chat online, Andrew brings up the point that clearly, we now have Marsh viewpoints, so we could definitely get a lot of information from there. However, if we are correct (assuming you buy the theory about the earring being Hemalurgically imbued), then Vin will need an understanding of Hemalurgy to release her full potential.
Who would tell her. No Lord Ruler... no Marsh... definitely not any Feruchemists, since Marsh purged the Synod. The list--at least of characters we know of--becomes to... none. Except... Vin, maybe?
My first thought was that perhaps since Vin didn't use the power, she wouldn't get that knowledge. But then I realize that it's the holding of the power that gives you the knowledge (possibly giving Rashek knowledge of Allomancy and Hemalurgy), not the use of it. In fact, if Rashek was consumed by guilt over using the power wrong, well, then Vin actually has it a little better. She only has to be consumed with guilt that she released an almost invincible force of nature (I wonder which shatters the mind more...). Anywho... she still held the power. Which means it is possible that Vin knows the answers already.
We won't know for sure until we get in Vin's viewpoint, but it's a theory all right.
Also, my undeveloped thought is that the Hero need not be someone specific. It could be anyone... even Rashek, but maybe he just didn't use the power right.
Finally, whoever had that conversation about astronomy and axial tilts of planets has my gratitude. I shall never mock such things ever again.