The Splinter of Ambition

Some characters that are simply Ferrings that have a deep understanding of their powers and could maybe teach a new full Feruchemist, like how the Mistings in Era 1 help teach Vin.

Mercy will definitely be the Shard I will be watching most for later in the cosmere, despite people talking about Valor and Reason I still remember Harmony saying Mercy "worries" him.

Double electrum looks interesting in combat as seeing what could happen to me in a fight would be useful with allomancy and compounding. Then I'd just tap feruchemical electrum to ignore/push through anything that could happen to me if I got overloaded with the information of my future through compounding. Imagine being like the clone troopers throughout the clone wars shows tanking several hits, BUT also having what effectively could be interpreted as Jedi foresight. EDIT: for just being a normal person and not some main character in a mistborn era I'd go allomancy brass, feruchemy tin.

I think a combination of Ruin and Odium would best be called "Malice", and would probably be the most terrifying Shard to ever menace the Cosmere. I'm imagining some pretty awesome gigantic beasts like thunderclasts but enhanced with hemalurgy...

First post/theory so please don't hate: I think that the first triple shard is gonna be Harmony, as I see Whimsy gallivanting around and some way Saze is going to take the Shard and become Discord due to the unpredictable nature of Whimsy combined with the conflicting natures of Preservation and Ruin.