My first post on the site. Hey everybody.
I don't think it's lerasium, but something that changes the person like lerasium does. I believe the element may change whoever holds all the shards into Adonolsium. Just as lerasium makes someone a mistborn. If we assume that Hoid's plans were a success in Mistborn the biggest change is that Sazed holds two shards. If Hoid's plan is as some assume to restore Adonolsium simply reuniting the shards may not be enough. Some other element may be key that turns the ultimate shardholder into Adonolsium. If Hoid is united the shards into one subject a catalyst may be required to take it to the next level and return his friend/enemy/frenemy Adonolsium. That, in my opinion, is the element. What else would be so important to Hoid and his plans?
Whatever the true identity of the element is it may be mentioned in another book. The only options are that Hoid obtained the element outside the pages of a Sanderson book, we see the plans of Hoid in another book involving the element, or we actually see Hoid obtain the element. It may have been somethign that seemed utterly trivial at the time. 2/3 isn't bad odds. Re-reading the other novels for some element may be worthwile.
Anyway, sorry about the spelling.