This wiki article does a very good job at explaining how it works so i would ask you to read this first before proceeding: http://coppermind.net/wiki/Metallic_Arts
As stated in the wiki article there are 3 metallic arts: Allomancy, Feruchemy and Hemalurgy. This would equal to three sub-classes ingame. Allomancy works by consuming metals, while Feruchemy draws from the powers of the metals worn on ones body and Hemalurgy steals the powers of other Allomancers by driving a spike through them at a specific point and into the recipient.
So rudementarily i imagine it like this:
Allomancers have a metal reserve which they draw upon to use their powers. It does not regenerate like mana, but you have to use the Transmogrification Chest/APE to refill it. It has to be a metallic object and the higher tier the object is, the more it adds to your metal reserves.
or
There could be a talnet tree which would be a mix of the tinkers Physics and Chemistry called Metallurgy. In that you get talents to do with crafting vials/potions which refill your metal reserves (with a max amount to be able to carry to not make it spamable ofc). This would also be closer to the original and I personaly like this better.
Feruchemists draw their metal reserves from the metal they wear. So I imagine it like this that they have skills that are dependend on what kind of metals they wear. For example if you want to use your copper skills (from the skilltree which uses copper) you have to wear something like a copper amulet. If you dont wear something made out of copper you cant use the skills for it, same with any other metal. So maybe Feruchemist players always have to have every kind of metalic item in their inventory like a set of steel/gold and copper rings for example. So in a fight if they want to use a skill but dont have that metal equipped they have to switch out what they are wearing, thus loosing a turn.
Hemalurgists draw their powers from the spikes they have in their body. I am not completly sure yet how one would go about aquiring new spikes with the current game mechanics. But lets pretend we already have a system thought out for that. In the novels Hemalurgists dont need to refill their metal reserves with anything externally like Allomancers and Feruchemists (among other cool powers). Though it is written that they need to rest after each fight so i guess each spike works like a battery. It could work like regular mana which refills upon resting.
I can already think of many cool/unique abilities but i think it would be better to see first if you guys share my enthusiasm
I would be happy to help in designing this if all mighty darkgod permits (though i dont know much about programming, im more like the cool ideas guy 
).Thank you for reading! I would be happy to read some opinions on this.
