I think the new FE will be like Radiant Dawn, in which you can play as both of the warring factions in different chapters, as in, you play as both the Western and the Eastern factions alternately. In fact, you can see the player using both Western and Eastern looking characters on the trailer.
Perhaps, the decisions you make as each faction will influence the story somehow.
I think neither the Western and Eastern factions will be villainous, which once again reminds me of Radiant Dawn, where you had Ike on one side and Micaiah on the other, each having their own motives to fight. There will, of course, be a third faction: the villains that are responsible for the monsters and what will eventually cause both warring factions to make a truce and fight the common enemy.
Just a theory that I came up with. And with this, I'm off and back to studying. See ya.