The four major clans ,besides Clan Mors or the Generic Skaven faction, should have differentiation's that would make them differ from each other and have their own playstyles.
One is for each of the four clans to have either discounts or increased cost's of their units. For instance:
Clan Skryre could have discounts to their weapons of war and their units who wield things like the Ratling gun or Warpfire-Thrower's and their weapons stats are better. Alternatively, their warpstone based units and machines will be even more expensive, but again their stats are better (and perhaps less likely to explode (or more likely or sumthin))
And perhaps like Middenland, each clan will not have access to each of the other clans goodies. For instance, Clan Moulder has their warpstone molded creatures, but don't have access to Skryre's machines. This could be a restriction to both campaign and multiplayer, or just restricted to multiplayer, and other clans can purchase each other's toys for warpstone in the campaign.
And concerning Warpstone, I have heard that people would like to see Warpstone as a secondary resource like the Wood Elves's amber. The Skaven could have this primary resource called loot/plunder, and the secondary one is either warpstone or warpstone tokens. The four clans can sell their weapons/units to the other clans for warpstone/warpstone tokens, only except they can gain more than the Wood Elves.
What do you guys think ?