After reading through all these lovely theories, I have to partially agree with Skaa about Harmonium being an alloy. However, I believe they are the alloy of the two god metals combined, rather than any other portion therein. If the 16 are all there are, then mixing any others together would not result in any effects, it would just cause an Allomancer to be sick.
However, if someone were to combine the two god metals (I don't know if it's been done or even discussed anywhere), it could potentially be the missing metal, Harmonium, which has no real information at all, though speculation is a great thing to play with.
Alternatively, this leaves me remembering the Pools left by the shards, and makes me wonder if, by any theory, the pools are actually created by someone becoming a shard bearer (not just a sliver). That could, for instance, be part of the reason that Rashek wanted to hide the Atium from Ruin, why when Preservation gave her life to lock Ruin away there was the Well of Ascension, and why, on Sel, there's the large lake that lets Elantrians return home. This could also be where the new "Harmonium" metal comes from, being a well of metals left behind when Sazed became Harmony.