So, we have Mistings, each with the ability to burn one of the Allomantic Metals, including atium (and its alloys?) and we have the mists, which Snapped mistings so that they could burn their metal (even the atium mistings, though they got it a little tougher, I guess cause they had 'more' Ruin inside of them?).
These guys have been round since forever, but before Rashek's ascension they were a real rarity, I guess cause the mist was unknown until the Deepness, which was a little too rough. But after the ascension, there appeared a number of new guys on the block: Mistborn, those that could burn all 18+ of the metals, were loved by the mists, and generally had it pretty swell.
As we all know, (at least to my understanding) the magic systems of the Cosmere are accessed by those with some sort of special 'spiritual DNA' or sDNA, which is pretty darn cool. I hear (an aside) that Roshar, (the planet that The Way of Kings is set on) has 30 different magicks, and yet Scadrial (Mistborn's planet) only gets 3? That doesnt seem right. Each one of these mistings can only use (burn) one of the metals, I dunno what happens if they try burning others, probably a nasty head/belly ache or just nothing at all. But yeah, only one? Sounds like maybe their sDNA is different between each of the metals (An aside: I'm now questioning this, since the table of metals says that spikes take a subset of Allomantic/Feruchemical powers. Maybe this just means 'can take one of the metals, depending on who you spike or where you place it?). So we have the overarching skills of a Feruchemist, 16 standard burning powers and 1 or more atium +alloys burning powers. That looks to me like 18+ sDNA's. Hemalurgy is stealing/twisting the sDNA of others onto another being which (I guess) 'creates' new types of sDNA.
"AHHA!" you cry, "but what about Mistborn, those who can burn all 16 metals?". Simmer down there, I'm getting to it! If you didn't realise I also have not spoken of lerasium, the metallic form of the god Preservation (counterpart to atium's Ruin). "But lerasium mistings make no sense!" you say, "as soon as one burns lerasium, they become full Mistborn!" That they do, my friends, that they do. But there were no Mistborn before the Ascension, (I believe) not even Rashek The Lord Ruler was a 16-burner before he touched the Well. The Well that was chocked full of Preservation's power. Only after this did full Mistborn appear, after he gave some of his friends the beads of lerasium, which any Joe can burn, and said Joe gets the powers of all Allomancy.
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As a counterpoint to my argument, Leras gave up a part of themselves to fuel sentience (or something akin to this), and all everyone can burn learsium (and not animals (? confirm)), so what if lerasium mistings are given sentience (or whatever Leras made in humanity)? or the fact that ....
Darnit, my string of thoughts has fallen apart, and my theory has many holes, such as tin mistings also getting mist-sight, and the mist not only liking mistborn/lerasium mistings.