Not long a´go I was with a friend of mine who plays medieval 2 total war that, strangely enough, has choquered the entire map with Scotland. So I was talking with him about this mod, and he gave me some good ideas:

-ORDERS. In medieval you have teutonic, templar, and holpitaler knight. What about: Orders for one of the four chaos powers: Khorne, Nurgle, Slannesh and Tzeentch, that would have thir own demon units and so on. An in the empire we would have it knightly orders:
http://uk.games-workshop.com/empire/knights-chapters/2/
The knights Panther, knights of the Blazing sun, the knights of the white wolf (also/specially for Kislev),the knights of the Black Bear, the Knights Griffon, the Hunters of Sigmar, the Grand Order of the Reiksguard, the Templars of the Everlasting Light, the Order of the Broken Sword, the Order of the Sacred Scythe, the Knights of Sigmar's Blood, the Knights of Morr, the Order of the Gold Lion, the Knights Encarmine and the Order of the Hammers of Sigmar.

-DRAGONS and MONSTERS. In medieval you have pirates, as well as in warhammer, but in warhammer you have sea monsters too, so sea monsters for sea battles.
Dragons are difficult to find but you may be surprised when a rebel army reaches you. He give me the idea of lonely dragons wondering around the Old World. They could be both, giant and little (the size of a horse) dragons. Big ones should be a threat for an entire army, while little ones could be dangerous four a pair of bad units, not as dangerous at all for a unit of slayers (Dwarfs) of course.

-MORALE. Of course, skelletons shouldn't flee.

-CASTLES. My friend plays with Bretonnia and he told me about the important castles surrounding the lands and about the castle, Bretonnian in origin, that is inhabited by the Blood Knights, the best knights of them all (if we don't take on account the presence of the Grail Knights. this Blood Knights could be rebel, or part of the Vampire Counts.

-KINGDOMS. He told me about the land southeast of the Empire, where there are a good number of independent kingdoms, they should be implemented as Rebels. A good thing for their near vampire Counts who own very little terrain.

-VC. Vampire Counts have little terrain, so, in order to not disappear in the first two turns, they should have a very big army or low maintenance costs.

-REBELS. Skavens have their terrain but they are suposed to live under the earth in all the world, so this factions, as well as others (orks, chaos...) should be in part rebel and spawn all over the campaign map.