My favourite undead group of all time is the Legion of D'tesh from the Rise from Erebus mod for Civ4. It's an entire civ founded on the basis of a single city. The city's population is increased by using slavery and turning the slaves into more legions of undead. Absolutely amazing to play. The Scions of Patria are similarly interesting, being undead in haunted woods and an extremely interesting faction based around a religion serving the Emperor and immortality.
If we're talking about just pure lore, probably Magnagora from Lusternia (a mud) because they're basically a combination of industrialisation and demonic worship. Oh, and about 75% of the population (in terms of players) are undead.
In terms of human cultures, I think we're missing a nature orientated culture, a nomadic / wasteland enjoying culture of some sort, barbarian raiders and some sort of Asianic culture. Out of those, I only think two and three could be considered "necessary" as a way of rounding out playstyles, though. Those are what I've enjoyed in other games, at the least, and what I see lacking in this one..
Favourite fantasy cultures: Dwarves in Warhammer and Dwarf Fortress, goblins / orcs in Warhammer, Dwarf Fortress and Lord of the Rings, EVERYTHING from the Exalted P&P.
Can't think of any more, really.