I can see his arc going in this direction. His story has dealt so much with how as Truthless, he has influenced Roshar which brought him into contact with Dalinar, Kal and the rest. His story will be centered around the Shin for the next book or two and be separate from the KR. This provides good symmetry with Kal - he has branched off and gone his own way. In terms of story, this obviously exposes us to greater Roshar and also the opportunity to learn about the Heralds and the Day of Recreance.
The ends of WoR has set things up so that a lot will happen before/if there is a meeting between Szeth and the others. And then, I just don't see Szeth going from a one-man-army to joining any group co-op. They may very well end up working towards the same goals, but I see this happening from different directions. If they were to come together during those efforts, then I could see them fighting alongside one another but not as a unified "team".