I read that scene as a warning that the Seanchan have to be dealt with by Rand, from a position of strength. I sort of assumed that they would keep their lands, and the other nations would keep theirs, and that would be that. But that's just naive. The Seanchan are aggressively expansionist conquerors, and very good at what they do. If they aren't stopped from leashing channelers, they'll win, pure and simple. They can't be ignored. And Rand is the only one who can do it. Perhaps that's Aviendha's purpose in the Pattern - to take him that warning.