I haven't made much progress into making the classes, ( I haven't put the most effort into it either in the last week or so) but the metal pools was the way I was planning on doing it, with different metals being consumed at different rates, and probably also adding a allomancy skill to see how much juice they can pull from their metals.
The books say there are 16 total metals, and the wiki and web state 18(8 lower, 8 higher, 2 god), I was leaning towards the second one, and both the lower and higher metals are split in to two groups on four, I was thinking I would make each group of 4 into a base class at first, but alot of the metals don't do much aluminum and duralumin have no use for mistings; electrum and gold have minimal to no benifit; I can't see much advancement for chromium(not in books), nicrosil(not in books), copper and bronze; while I can see room for bendalloy(not in books) and cadmium(not in books) I can't see them being a 20 lvl class. I'd also want all characters to be able to dabble into allomancy without a serious lvl comintment so I thought feats may be in order, as of now through a lesser bloodline feat, that would allow you to take a feat to use one of the lower metals to some small benefit, or take a stronger bloodline feat which would allow access to the higher and atium.
So I came up with a break down like this
Physical Misting (Pewter, Tin, Steel, Iron) 20 lvl Base Class
Emotional Misting (Brass and Zinc) 20 lvl Base Class
Temporal Misting (Bendalloy, Cadmium, and maybe atium[also the only class I have 90% done]) 5 lvl PrC
Enhancement Misting (Nicrosil and Chromium) 3 lvl PrC
Divining Misting(Copper, and Bronze) 5 lvl PrC
Mistborn 10 lvl PrC
Ferchemist 20 lvl base class
Keeper 10 lvl PrC
Gold, Electrum, Duralumin and aluminum mistings would be a second or third tier feat.
For class features I'm thinking of taking a more D20 modern or ToB look at it and makeing trees in each class that they pick which abilities they gain from the class, with mistborn having access to all trees(and abilities granted from the misting feats), but making them weaker in any one than a specialist misting in that field.