My guess is a lot of 3-4 cmc Planeswalkers and a lot of power behind them. They have been building up to this set for a while now with the Gatewatch. Ravnica, Innistrad, Kaledesh, Ahmonket, Ixalan... YEARS of story build up, to a climatic confrontation. So I think there will be a lot of Planeswalker support and Planeswalker kill spells as well. Legends designed specifically to eliminate a Planeswalker in specific circumstances. I think all the spells that aren't Planeswalkers will be tied in to one or more of the Planeswalkers via flavor text or artwork.