The problem thinking in big empires is because usualy we take the earth experience as reference. The realms in our world take the religion as reference, everthing come after this. Big empires kill in name of god, big religious empires start to tell about god plans and take the ppl folloing the same reasons. The economic and politic can not avoid from religion, because liking or not the faith is the main power to manipulate the people.
Look something like Avalon, all empire of is moving about a religious war around Camelot. Now comes the the point where i really like. Remove religion from the heart of ppl and start to think about survive or just power. The world of Mad Max or Waterworld, where the empires are strong, given wut they can compare, but still ruling some area.

Like Coulin said is prior think how they will survive in the ice, farms can be a problem, probly they will hunt meat moving one place to other. I believe magic will not change the goals, since they are looking for survive. Nomads people just go to some place, if it is occupied they kill and use all sources before move again. Till someone appear and start to tell about evolution (nature or political).