I would like to see a legitimate Nordic faction as well. I hate, hate, hate that the Sturgian special troop is mounted considering people during the Viking age would use horses to ride to battles and then dismount to fight. I would love, love, love for there to be a troop tree with Houscarls instead of mounted skirmishers. Maybe not an entire new faction but one of the minor groups. I believe there already is a Nordic inspired minor faction. Maybe even add another one to one of the Empire factions similar to the Varangian guard.
It would also be pretty badass to get a North African or Indian inspired faction with War Elephants.
I'd imagine a Japanese inspired faction with Samurai would be fairly popular as well.
Maybe even create some Aztec or Iroquois factions.