So this may have been hashed and rehashed a thousand times already, but I need to put it out here just in case.
I just finished rereading the Way of Kings and I came across a theory. That theory is based on Dalinar Kholin's first (and last) vision showed to him by the Stormfather.
So here it is: I think that Honor is not asking Dalinar to unite Roshar. I think he is asking Dalinar to unite the shards of Adonalsium to overome Odium. I haven't yet decided if that means recreating Adonalsium or if it's just a unification of intent, but that's my thoughts.
Please feel free to roast away if this has been addressed or if it sounds silly.