Treachery, Deceit, Honour, Friendship.
As the players take on the roles of a band of brothers (and sisters) who've grown up together, friendship is an obvious theme. The PCs should be able to trust one another with their lives, and possibly be willing to die for one another.
Treachery and deceit are the twin evils that have brought the Realm low of late. Once loyal kings and chieftans have begun to break themselves off, refusing to pay taxes, even becoming hostile to envoys from the crown.
Honour is part of what should drive the PCs, or at least some of them (there will be no alignment restriction) to fulfil their quests. Likely they will be forced to make common cause, though their motives and modes might differ.
I hope to include some pitched battles along the way, with the PCs playing key parts in deciding the victor through specific missions in the midst of a raging battle, as described in "Heroes of Battle", one of the best supplements I've seen for 3.5.