Just finished reading Shadows of Self. Will try to hide spoilers, but the revelations at the end of the book are key to the question.
So at the end of the novel, the kandra Paalm was shown to be using a Hemalurgy spike of a new kind of metal unknown to any of the other characters. In fact, it's unknown to the kandra and Harmony as well, and it's called out as a new type of god-metal, one that definitely didn't come from Harmony. Properties unknown, but the initial theory that Paalm was using multiple spikes to switch Allomantic and Ferchemical powers is kinda put off by the fact that they only find the one.
This points to another Shard holder from elsewhere in the Cosmere either present or trying to interfere on Scadrial, the Mistborn world. The problem is, Shards don't hide very well. Harmony should know if there was another god about, and he's got nothing.
We've only seen this type of physical manifestation of Shard-power on Mistborn's Scadrial though. Every other power that we've seen in the Cosmere manifests in a completely different way - spren and stormlight on Roshar, aons and the Dor energy on Sel. The only point in common to any world is some kind of pool of power. We know there are people that can hop from world to world (hello again Hoid! cheeky bastard), and the way abilities interact can reveal new secrets, but the abilities themselves never seem to change their base nature (Hoid alludes to the fact that he has Breath from one world in him, but it doesn't suddenly transmute into Stormlight just because he's now on Roshar).
So how does this new metal suddenly appear? Curious if Sanderson has said anything, or if there are other hints in other novels I'm missing.