I thought of some interesting themes to suggest...but I think you already have them.
Pangea: All animal units, maybe werewolf mages...DRAGONFLY SCOUTS!!!
and an undead theme where you pay gold and resources to build the undead?
Also, perhaps an all etheral theme...it might be hard to do though since most etheral units are undead or very powerful.
A theme involving the aesir somehow perhaps...vanheim with giants.
And a nation where every unit has a bow and good precision...."Reign of Arrows"
Maybe for caelum.
Oh..and I want the Black Coven too...I'm thinking Ulm with death/earth mages and unholy priests maybe. Now that would be a cool Ulm theme.
Just imagine if they could get wights with black steel plate on.
Be really cool if they reanimated undead with black steel plate.