I like the winter is coming theme but I think that excluding the winter based heroes is a bad idea. As for some of the more "summer" themed heroes like Lina, don't necessarily think of blue/white/icey colours instead think of what those heroes would wear during winter - furs, thicker clothing etc.
It's almost winter, so how about going for a ice themed month? We could go with heroes (Drow, tusk, cm, lich, abaddon, invoker (had to stretch to fill with more), or make winter clothes for any hero
How about "Winter is coming" but with the limitation that you can't use ice heroes? So no Crystal Maiden, Drow, Lich or Tusk (think those are the only ice heroes on the workshop at the moment)