Civilizations from previous games that are not in Civ VI:
Babylon (I, II, III, IV, V)
Mongol (I, II, III, IV, V)
Zulu (I, II, III, IV, V)
(The other 11 civs from 1 return)
Carthage (II, III, IV, V)
Celtic (II, III, IV, V)
Sioux (II)
(Japan, Spain, and Persia return from II in VI; you could also count Norway as II's Vikings)
Iroquois (III, V) (IV had Native Americans)
Hittites (III)
Korea (III, IV, V)
Ottomans (III, IV, V)
Byzantine (III, IV, V)
Dutch (III, IV, V)
Incan (III, IV, V)
Mayan (III, IV, V)
Portugeuse (III, IV, V)
(Arabia and Sumeria are the only returning civs from III in VI)
HRE (IV)
Khmer (IV) *
Mali (IV)
Ethiopia (IV, V)
(No civs introduced in IV have returned for VI)
Assyria (V)
Austria (V)
Danish (V; was pretty much Vikings in previous games, akin to Norway in VI)
Huns (V)
Indonesia (V) *
Morocco (V)
Polynesia (V)
Shoshone (V)
Siam (V)
Songhai (V)
Sweden (V)
Venice (V)
(Brazil and Poland are the only civs introduced in V to return in VI so far)
Total returning civs from each game: 11 from I, 4 from II (if you count Norway as Vikings), 2 from III, 0 from IV, 2 from V
New to Civ VI: Kongo, Scythia, Nubia, Australia, Macedonia (3/5 of them from DLC)
Civs that have returned for every game since their introduction except VI (not including civs introduced in Civ 5:
I: Babylon, Mongols, Zulu
II: Carthage, Celts
III: Ottomans, Byzantine, Dutch, Inca, Maya, Portugeuse, Korea
IV: Ethiopia
The only civs that are not from Civ V and are not a "conglomerate" civ like "Vikings" that have only been in one game are the HRE, the Sioux, the Hittites, Mali, and the Khmer (for now, at least).
The only civ that has missed a game and then returned in a later one, and isnot yet in Civ VI (ie: Sumeria) is the Iroquois (even then, Civ IV had the Native Americans, so it's a bit debatable). The other 13 civs have not been dropped from a game in the series once they have been introduced.
Representation-wise, the Sioux and the Shoshone occupy a similar geographical slot; so do Denmark and Norway, which is already in VI. I think it's fairly reasonable to assume that we'll see all of the consistently-returning civs as DLC (Babylon through Portugal above) as well as possibly the Iroquois. That + the two upcoming DLCs makes 40, which is still less than the total civs by the end of Civ 5. Of course, this would still be lacking a NA Native American civ besides the Iroquois, a western and/or central African civ, and some sort of Polynesian civ, which they might split like the did Vikings and Native Americans in previous games. With Sumeria and now Khmer, Civ VI has already pulled two civs from games earlier than its immediate predecessor, the same number that Civ V did itself. Add in some strong potential Civ V returnees like Austria and the Huns, and we still have a handful of empty civs before we hit 50. (Hypothetically speaking, if they added all 14 likely-returning civs + second NA civ + west-central African civ + Polynesian civ + Austria/Huns, it would total to 45).
The options they could realistically go with for the NA civ(s) are:
* Introduce only one NA civ, whether new or returning (least likely by the end of Civ VI's life, though obviously somewhat likely in the short-term)
* Bring back two between the Iroquois, Sioux, and Shoshone--if this is the case, Iroquois/Sioux seems very likely; Civ VI seems to be bringing back older civs that missed Civ V, of which the Sioux definitely fits the bill, while the Iroquois have already been in multiple games at this point
* Bring one back and introduce a new one; the Shoshone seem the least likely to return for the above reasons, but whether they pick Iroquois or Sioux for the returning one would be a toss-up
* Eventually have three NA civs; this is probably unlikely, but possible by the last of the Civ VI DLC; I could easily see two returning NA civs and a new one if this happens
Between the reasons already in the OP and just looking at the number of returning civs and what games they came from, the Sioux are definitely a very strong candidate for DLC; if there end up being 3 NA civs, even moreso.