I have a suspicion that it is going to be Moash.
There is a scene in WoK when he is practicing with the spear in a chasm, by the light of a dozen spheres, and he is described as having an equal number of shadows. I know that it is far from definitive, it may not even be foreshadowing since the number of shadows is different. Its just a scene that made me think of the possibility, and after pondering the idea, it seemed to fit. While he and Moash were friends, they were also opposites. Kaladin wanted to excel at fighting in order to save people, Moash to kill them. I could keep listing examples...but the list goes on. Btw...he also has shardplate.
As the singular connector pointed out, Brandon likes to develop his important characters. It would be an interesting plot development to have these two, once friends, stand toe to toe, in the battle for roshar.