Again, I can't help but coming back to that lake, with the metallic sheen, it seems to planned to be jsut thrown in for kicks, and I don't think it should be ignored. I thought it was where the metal that made allomancer came from, but I guess it could also be the alloy.
Hmmm. Perhaps the 15th metal is the one Elend ate, which gives powers of Mistborn, and the 16th is the metal at the Well of Ascension?
I like that theory, but it doesn't seem quite right. For each Allomantic "quartet" (Physical, Mental, Temporal), there are two that are External, two Internal. I expect that the Aluminum quartet would have the same properties. The way I'm reading it, aluminum looks like the Internal pair.
Those 15th and 16th metals would need to be external, right?
Oh, and here's something Brandon posted on the fan art thread. It certainly verifies that there are two more metals.
http://www.brandonsanderson.com/graphics/Mistborn%20Wallpaper%20One%201028.jpgA couple of things about that wallpaper:
1. It's really cool. I need to set it for my desktop.
2. Remember, the Internal metals are the inner circle.
3. Notice the dots on each of the pairs. On one, the dot is inside the rest of the symbol, and the other, it's on the outside. I believe this signifies which metal is a Pushing or Pulling metal.