Sumeria was included in CIII and CIV, although never in vanilla.
As the cradle of civilization, Sumeria simply must make a comeback!
CVI without Sumeria/Sumer would be like the Summer Olympics without Track and Field.
I'd say Sumeria should be in CVI Vanilla, to make up for its omission in CV.
Sumeria:
Leader: (most likely) Gilgamesh
UU: Phalanx
Sumerians invented the phalanx formation, this unit should be available for them early in the game, be stronger than generic Spearman but certainly not as powerful as a Greek Hoplite.
Hittites (Hatti)
Featured in: CIII
This, once great and original civ, mysteriously disappeared from both, CIV and CV.
They absolutely should make a comeback! In vanilla CVI won't happen, but at least in one of the expansions.
Great Leader: Mursili, Muwatalli or Suppiluliuma.
UU: 3 man chariot
Vikings (Scandinavia):
Featured in: CII, CIII, CIV
Although by definition not a single entity-not a single nation, Vikings made great contributions to the history of civilization, by their advancements/inventions in ship building- longships capable of river and/or ocean sailing (along with warfare and trade routes).
To me it's clear they should in some way make a comeback, perhaps by limiting other Scandinavian nations, such as Denmark and Sweden from being available (for AI, or other human players in multiplayer) in the game if their forefathers, the Vikings are selected.
If not adding back Vikings, Firaxis should at least introduce Norway as another Scandinavian option (in one of CVI's expansions)
Vikings:
Leader: Ragnar Lothbrok(Lodbrok)
UU: Longship (Longboat), Viking (or Berserker)
Sioux:
Featured only in CII up to this point. Replaced by another North American native Tribe: Iroquois-in CIII through CV.
Hope to see them one day make a comeback.
One of the more significant, large and powerful north American native tribes of history.
Sioux:
Leader: Sitting Bull
UU: ? (Dog soldier/warrior was a Cheyenne unit, so not sure about this, but could be: Mounted Brave)
Holy Roman Empire:
Previously featured in: CIV
Due to the fact that this empire was vast (in European standards) and lasted for several centuries it should make a comeback.
It's main great leader: Charlemagne, is one of the more famous (European) great leaders of all time.
Native America:
Featured in CIV
Such mixture of tribes, most of them with a separate language, traditions and customs was bound to fail in the eyes of the players.
However, I still think the concept was good, by giving their cities north American native tribe names I was able to a some degree, have in the game, the Huron, Seminole, Apache and Powhatan (among several others), something I will never get in a real game because each of these tribes will never feature in the game by themselves (I think)
Mali
Originally featured in: CIV
This is 1 of my favourite tribes to play as in CIV, would really like them to make a comeback.
Khmer:
Originally featured in CIV
Khmers should come back, in one of the CVI's expansions, their unique history (and UU in CIV) are deserving of this.