Babylonians - No brainer
Turks - likewise
Norse - Fun Factor
Byzantines - They're already in a scenario, might as go all the way
Maya - Mesoamerica is always cool
Khmers - Southeast Asia is always unrepresented in Civ. If you have Angkor Wat in the game, you might as well have the people who built it.
Thai - To go along with the Khmers
Ethiopians - Xen's post essentially sums it up
Dutch - As evil and reviled as eurocentricity is, The Netherlands deserve a spot in game
Portuguese - See above
Koreans - Another east asian power would be nice, and it makes sense from a sales point of view.
Brazilians - If America is in, other post-colonial civs deserve a spot as well, and Brazil's the next best choice.
Carthaginians - Who doesn't want to recreate the Punic wars?
Hawaiians - Yeah, Hawai'i didn't have a huge impact on world affairs, but it's different, it fills up an empty area of the map, and I think it'd be fun to play as.
Iroquois - Can't forget the Native Americans. The Iroquois Confederacy is the closest thing to a state you could hope to find in pre-Columbian North America. If they were good enough for Civ 3, they're good enough for C-IV.