15th metal - This is Preservation. There are many metals in our blood, some of them more prevalent than others. So, my thought, in an attempt to try and keep the world together, this metal runs through peoples blood. This could explain why you have to be noble, because it could be a metal only inherited from a parent. And different concentrations of it, or different blood types, could react differently; thus causing Mistings and Mistborns.
The only problem with that is that your parents wouldn't be able to pass that much of the metal on to you, and after you were born the only way for you to get more metal is to take it from your parents blood (assuming of course, that there is no other source of this metal.). Because of that, the number of mistborn/mistings should have decreased at a far greater rate than it did.
16th metal - This is Ruin. This metal wants everything to die and perish. Thus it takes on the form of mists. Under the Lord Ruler's reign, he could control the mists, thus keeping them at bay. Not that there is no one to control them, they have started bringing destruction about again. The voice could have been the essence of the metal, which could have taken a physical form, reveling in the fact that it can now reign free because they broke a barrier set up by the Lord Ruler. The thing that the Hero of Ages SHOULD have done, was use all of the power he had found to destroy the 16th metal, thus causing preservation forever. The thing being that the Lord Ruler, wanting the power for himself, simply captured its essence and kept the power for himself.
So what you're saying is that the hero of ages should completely destroy Ruin? The problem with this is that then you are left only with preservation. Therefore, nothing changes, and the world slows into stagnation. There has to be balance for the world to work.
Take this into consideration - Vin even talks about burning the mists as if they were a metal. Steel and iron push/pull on other metals. What if burning the mists also gave you some control over the mists. This would explain why the mists acted like they did when the Lord Ruler was in control. He could keep them at bay during the day, and then allow them to keep Skaa in line at night. It could also explain the way in which the new entity was kept at bay.
So wait...You're suggesting a Third Entity that we haven't heard about yet? That seems to me like an unnecessary complication. It seems to me that Brandon would have mentioned a third entity if it existed.
Also, the ting about burning the mists allowing you to control them is plausible, but I don't really agree. Vin burns the mist, agreed. Brandon establishes that this is related to allomancy. The mists are, themselves related to allomancy. The problem is that the mists are (according to Brandon from his MB 1 Annotations Ch 38 pt 3) related to the mythology of the world and the magic systems on a foundational level. The mists are affected not only by allomancy, but also by hemalurgy, as evidenced by the inquisitors, Vin, and Zane. So I see them as being a kind of transition area between the systems, and also between Ruin and Preservation. I think that the Deepness was a result of Ruin controlling the mists, and now that Ruin is free, it can control them again.
If there was doubt as to the fact that the force released at the Well is Ruin, it is cleared up here. Product Description for HoA, Amazon.
Who is the Hero of Ages?
To end the Final Empire and restore freedom, Vin killed the Lord Ruler. But as a result, the Deepness---the lethal form of the ubiquitous mists---is back, along with increasingly heavy ashfalls and ever more powerful earthquakes. Humanity appears to be doomed.
Having escaped death at the climax of The Well of Ascension only by becoming a Mistborn himself, Emperor Elend Venture hopes to find clues left behind by the Lord Ruler that will allow him to save the world. Vin is consumed with guilt at having been tricked into releasing the mystic force known as Ruin from the Well. Ruin wants to end the world, and its near omniscience and ability to warp reality make stopping it seem impossible. She can’t even discuss it with Elend lest Ruin learn their plans!
The conclusion of the Mistborn trilogy fulfills all the promise of the first two books. Revelations abound, connections rooted in early chapters of the series click into place, and surprises, as satisfying as they are stunning, blossom like fireworks to dazzle and delight. It all leads up to a finale unmatched for originality and audacity that will leave readers rubbing their eyes in wonder, as if awaking from an amazing dream.
Emphasis mine, of course. But you see what I mean.
Also, I'm still confused, in what way would the Lord Ruler have used the mist to keep the skaa in line at night? Unless you were simply referring to him planting superstitions among them?
We've discussed the likelihood of the mists being metal, we never really came up with any conclusions, as I remember.
Here's a question: What would happen if someone who is already a misting or a full mistborn ate the 15th metal. PErhaps that's why inquisitors are such good seekers, and why they are seekers before hand. You take a seeker, give him the metal, he becomes a msitborn with great seeking abilities. Then add hemalurgy, with the spikes and whatever that does. Then again, we know Vin's great seeking abilities have something to do with her earring, which makes the above theory unlikely doesn't it? Oh well, jsut me thinking out loud.
And sorry Miyabi, I'm still not buying the Ruin and Preservation are metals thread, although I agree with what you have to say of how the powers are passed on and such, with the metals in the blood.
What if there were misting-like feruchemists as well, say ones who could only store one attribute. Maybe the fact that the powers split up after a few generations is something unique about allomancy. And Hemalurgy isn't genetic at all, it's only present in those given it by some sort of ritual. Anyways, just some particualr random ramblings that came to mind.
I was actually wondering about the what would happen if you fed a mistborn the 15th metal thing earlier. My guess is...Absolutely Nothing!!! As a side note, I can't recall ever having confirmation that Vin's seeking abilities are due to her bronze earring, just our wild speculation that seems most likely to be right.
eh, sry i was doing like three things when i wrote that, forgot all about TLR's culling of the Keepers 
Still, it would be nice to know what prerequisites are neccesary to use hemalurgy. After all, surely there must be more than simply making sacrifices and sticking metal in yourself.
Perhaps Allomancy is required? The only Hemalurgists we know of are also Allomancers.
The only Hemalurgists we know for sure that can use allomancy are Vin, Zane, and Marsh. And the Lord Ruler. I might have missed some. I agree that the others are likely allomancers, but I hesitate to just establish that as fact.
I'm continueing this thought on the hemalurgy thread.
I shall follow!
