The key, I believe, is that while Allomancy's power is based upon metals, its EFFECTS don't have to be about metal. Sure, pushing and pulling do, but pewter enhances your body, and I highly doubt that it does this by affecting metals. It's even less likely in one like tin, and emotional Allomancy? That's about the furthest possible thing from a metal as possible, the abstract thing called emotions.
All of Feruchemy is storing decidedly non-metal attributes.
Hemalurgy is metallic because we place gigantic spikes inside of someone, not necessarily that it directly affects metals.
Now, I have no idea how Ruin and Preservation exert their will (methinks it involves the mists and how it relates to the magic systems, somehow) and whether its Allomancy or not. It's far beyond Allomancy, so I believe this is above the three magic systems in some manner, or a distinct application that has not been envisioned yet (again, the mists are very mysterious in this matter. We don't know how they operate, much in the same way as we don't know how R&P work. We know that Ruin and Preservation are intertwined with the magics somehow, and we know the mists are as well. The thing we don't know is why.)
As things stand, "nothing" is still the most coherent thing the obsidian does.
Quite right.
On the other hand...
It could magically open the door to the ashmounts so we can get to the lake where the last treasure is kept.
I am not opposed to primary villains being thrown into lava, lava in the scene of a battle, or Vin falling into the lava and the real Hero of Ages, Emperor Palpatine, comes into play and saves Vin by turning her into Darth Vader.
Anything with lava, really.