I really like the idea of the campaign system. Especially with small goals such as magical items as well as the normal grab for land style.
Other goals/objectives could be things like if you control the province of 'x' you can create an alliance with the 'x' tribe that allows your army to pass through without triggering a fight.
You could also have terrain effects on your troops for the battle. If you have rested before the battle you may get an advantage in some respect, but if your army has marched across rocky crags and gone straight into battle, it may be disadvantaged due to fatigue etc... just some ideas floating around.
Cheers,
Ben.