Lets say... Sodium-NaK, Potassium-KLi, Lithium-AlLi, Rubidium-?
Next set of 8... Magnesium-Elektron, Beryllium-Spring copper, Calcium-CaSi, Strontium-AlSr
That metal will be NaK, an alloy of Sodium and Potassium. That's three, folks, with 13 left to go, so help me out here!
Edit: Actually, we really only need 6 more metals, and 7 alloys.
How 'bout a shard called Integrity? Where your magical power increases slowly by honest acts and drops a level if you do something bad? (I know, it's very Mormon of me to suggest that, but hey, Brandon's Mormon too.)
I had assumed that Vasher's four categories were just in-world explanations that don't actually have to do with how the magic intrinsically works. Therefore it wouldn't matter.
Potassium should be another of the 16 Epic Metals. Its allomantic power is that it lets you inspire/envigorate conversations, and its feruchemical power is that it stores whatever the opposite of boredom is. So when you store, you get really bored, but when you tap, you're not bored at all. So a K misting is a chatterbox and a K ferring is an apathetic.
DIEDIEDIEDIEDIEDIEDIEDIEDIE!!!!!!!!!! OBEYOBEYOBEYOBEYOBEYOBEYOBEYOBEYOBEY!!!!!!!!!!
AND BRING THEM WAFFLES! WAFFLES!WAFFLES!WAFFLES!WAFFLES!WAFFLES!WAFFLES!WAFFLES!WAFFLES!WAFFLES!WAFFLES!