In reference to Trell, assuming he's actually a shardholder, and not some other power we haven't been introduced to yet, my own opinion is that he's a new shard we don't know yet.
People say "rust and ruin"
The new metal had spots that look like rust
Warriors in red, again, seem to link to rust
What if there is another shard, Corruption, held by Trell? That may not be what Sanderson's calling it, but Corruption seems to work pretty well, given what Bleeder did to the people/zombie things.
Edit - Corrected "Shardbearer" to "Shardholder". Slip of the tongue, so to speak.
