perhaps the alloys of physical & cognitive metals steal allomancy & feruchemy, but, if coppermind is correct, there is an element/alloy pairing that steals hybrid/temporal powers & spiritual powers. my money is electrum stealing temporal allomancy to fit that pattern (aluminum to steal allomancy ehanchement powers, duralumin to steal spiritual feruchemy, gold to steal hybrid feruchemy). That leaves Nicrosil, Chromium, Bendalloy, & Cadmium.
Other traits that would be stealable??? I'm thinking Temporal Duration (maybe Cadmium), Connection (perhaps from Chromium). Nicrosil is so Investiture heavy, maybe it is needed to steal non-Scadrial Investiture?