Here is a list of Warhammer barbarian cities andunits i see could suit very well (note that each city is linked as an enemy of every playable civs to fitt fluff and warhammer unique flavour well)
_Barbarians cities:
Sartosa: Pirates
Ungol Tribe: Mounted barbarians
Lahmia: Barbarians and Lahmian vampire
Skytitans Palace: Giants
Lost City of the Old ones: Beast (dinosaurs ?)
Strigoi: Barbarians and Strigany vampire
Amazons: Barbarians (female barbarians would be nice)
Marienburg (good idea ? I don't know): barbarians and ships
Vampire Coast (Undead, undead ship and a carstein vampire
_Dens: (not writting the classical animal den already in)
Tainted Wardstones: Demons
Hard stone: Beastmen
Forgotten battlefield: Undead
Greenskin camp: Orcs and gobs
Raider warcamp: barbarians, mounted barbarians and mutants (chaotic style)
Lost Temple: Dinosaurs
So for land barbarians we can have:
chaotic style early warriors and horsemen, mutants, brigands, chaos spawn, weak minor unafiliated demons, unpaid mercenary bands, trolls, orcs, gobs, beastmen, skelettons, zombies, ghouls, fimirs, pisceans (savage fishmen) and pygmans.
Beasts:
Giants spiders, scorpions and rats , cockatrices, harpy, wild cold ones and terradons and maybe even swarms (bat, snakes etc...) + the animals already in the mod.
For sea barbarians let's take manowar sea monsters:
Ptolemean crabs,black leviathans, sea dragons, sea snakes, giant turtles, megalodons, krakens and pirates.
For Barbarian boss i can see:
Strigany vampire and necrarch vampire, wild carnosaurs and stegadons, sea king (the manowar monster like poseidon), chimeras, griffins and...dragons !
The more the variety, the better the immersion and the replayability i think
We could even put events like orthus invasion in FF. These events could be:
_Great cataclysm (1st turn of game): Lots of minor weak unafiliated demons all around the map.
_Nagash great Curse :Undead invasion
_Tong tribe horde : Huge horde of barbarian horsemen: the tongs are a human tribe living near the northern pole, During an invasion they almost wipped kislev and the kurgans and where the cause of the chaos dwarves veneration of hashut (they falled in chaos to be saved from the tongs).
_Skavens invasion: Plague and rat swarms invade cities before a lot of barbarian skaven units spawn.
A lot more events can be found includind early greenskins waagh and such.
I Hope it helps