I would like to take the time to again warn that this post CONTAINS SPOILERS. It also contains some of my own theories. I will outline why I think there is something wrong with how Atium works in Allomancy and give suggestions on how I think it should work.
If you find any mistakes in my logic or you have a reference that discredits one of my assertion please comment with it. This theory is something I want to be proven wrong on. Brandon's Allomancy/Hemalurgy/Feruchemy magic system is something I absolutely love, yet I cannot reconcile the issues I've found.
Basis/Lore
I feel that the way Atium works is incongruent with the logic of how Lerasium would indicate it would work.
In the beginning it was said that humans did not exist on Scadrial, Ruin and Preservation had to work together in order to create them. Preservation had to promise Ruin that she would let him destroy the world (as is his nature) in the future if he would work with her this time. It was also said that Feruchemy came ages before the other metallurgic arts. I think that Ruin and Preversation created Feruchemy as a consequence of their collaboration.
Allomancy (Net positive): Preservation’s power arrived during the first ascension, through Rasheck sharing the Lerasium beads with his most loyal companions. Allomancy generates greater force/energy than what is consumed from the metal.
Hemalugry (Net negative): Ruin’s power and would be innate in the world. Some of the power is lost when using hemalurgy, the one with the spike always has a weaker version of the ability than it's original owner had.
Ferucehmy (Net neutral, but not net zero): Exists through the merging of the two powers. Weak now, strong later.
This is where I start to see the incongruent behaviour.
Lerasium is a god metal, and everyone is capable of consuming it; Skaa or noble alike, and grants full-fledged Mistborn-dom. Brandon said that being a god metal, Lerasium has special properties in how it rewrites your "spiritual DNA".
I feel that Atium should be the same way; everyone should be able to consume it as it is also a god metal. Atium and Lerasium are the physical manifestations of shardic power, and like actual shards they should both also be able to be picked up by everyone. This is probably the reason behind why Lerasium can be consumed by everyone.
Atium’s Allomantic Effect
Firstly, Preservation was attributed greater foresight. This is how Preservation was able to win out in the end and not have to give in to Ruin. Thus, why is it that Atium gives foresight as an allomatic effect?
Secondly, if Lerasium’s allomatic effect grants the power of net positivity, then Atium’s allomatic effect should grant net negativity.
I think that Atium being net negative, it would possibly have these effects...
1) if you are a Mistborn it would turn you into a normal human being.
2) if you are already a normal human, it kills you as ruins nature is destruction.
3) If you are a Misting it would turn you into something akin to a spren with a nature that would be dependent on the type of Misting you were.
4) If you were a Ferring, it would only partial destroy you, leaving only a fragment of your physical/spiritual/cognitive aspects behind as a shell of your former self, which depends on the type of Misting you were.
5) It’s hard to say what you would be if you were a Feruchemist and burned Atium. You could be destroyed, as your net negativity would be greater than your net positivity, but it could vary. An interesting idea would be that you cease to exist but the three aspects of you (your physical/spiritual/cognitive aspects) would be fractured into separate beings, locked as metaphysical building blocks across the three realms.
Atium-Lerasium Alloy
Thus, the alloy of Atium and Lerasium would grant Feruchemy and not the ability to burn Atium as a Misting; as in the the creation of humans the merging of the powers created net neutrality.