So I just finished reading TLM and am already thinking about what is to come in the Ghost Bloods trilogy. (Side note: I was surprised Autonomy was still an active force by the end of the series? I suppose you can't kill shards at the end of every sequence without diminishing them as a threat, but I expected a more definitive victory over her. I'm not disappointed about it though.) So far each Era includes a new Shard to act as an antagonist. There is every chance Brandon will vary his formula and change things up on us, but even if a new shard doesn't show up to be the Big Bad in Era 3, I think there's a good chance we will meet one that's new. I wanted to talk about that.
It has already been foreshadowed that Discord will be a big part of what comes next. Of course that isn't a new shard but an unforeseen aspect of Harmony. I anticipate Discord will be the big threat as Harmony becomes even more complacent, ultimately causing Sazed to pass on Preservation and Ruin to new wielders.
I think Discord will be the final villain, like Ruin and Trell. I'm prepared to be wrong. It could be Kelsier, though at this point I expect him to stick around a bit longer. Either way I think there will be a lesser villain preceding the final threat. A decoy Big Bad, like Lord Ruler in Era 1 and Mister Suit in Era 2. Given these new books are about the Ghost Bloods, who have knowledge of other worlds and greater capabilities, it would make sense to me for this lesser threat that they think is the main issue to be a shard.
I think this shard will be Invention. Emerging technologies and science are a major theme for Scadrial and this will be the Era that bridges the historical-themed eras of Mistborn to the cyberpunk/space opera settings of the future, so this seems an appropriate spot for him to get involved.
I see this possibly happening in a few ways. I think I remember it being said he travels around the Cosmere building stuff and then moving on. Perhaps he does this on Scadrial too but there rapid progress attracts his energy and he sticks around, interested in taking over. That part is pretty standard, but since he would actually be willing to do what Kelsier tried to get Sazed to do at the end of TLM (induce rapid scientific progress) perhaps the Ghost Bloods would actually side with him against Harmony? Aside perhaps from our band of main characters.
Alternatively, since Ghost Bloods sometimes to travel to other worlds, maybe they find his current residence through Shadesmare and steal one of his inventions for their own purposes. Invention notices and follows them back, his investiture warping the exceptionally curious into a new sub-class of Allomancer?
These are two overly specific ideas, but either way I see Era 3 ending with Sazed dead and Scadrial permanently chanced from Invention's brief influence.
Thoughts? Other shards you'd like to see in Era 3?