I'm pretty sure there aren't any additional base metals... regardless, we know that silver is totally useless, allomantically.
That said, it might be fun to work up an alternate set of metals. 'Fogborn' or whatever, where the little known shards Perspiration and Loon have attempted to create their own magic system.
Loonium: Kills the user
//Devnotes: Aw crud. Must have misplaced a negative sign somewhere. It was supposed to turn the user into an unstoppable killing machine.
Gatoradium: It's got electrolytes!
//Devnotes: Wait, what do you mean that doesn't let you zap people like palpatine? Outrageous!