The only DLCs I really care about are those with Civs. So if Civ6 has no expansions and it's all DLCs, here's what I think about the possibility of these Civ5 civs returning as Civ6 DLCs:
Assyria-possibly, may not return this time, why not put Semiramis for the lols
Austria-likely will return, with Maria Theresa again
Babylon-definitely will return, would be a shocker if it didn't, I bet Hammurabi will be in instead of Nebby
Byzantine-definitely will return, with a female leader
Carthage-definitely will return, has been in the franchise since CivII
Celts-will return in some form, whether as the Gaels/Gauls, a single blob, or Ireland/Scotland/Wales
Denmark-probably will return, not with a Viking focus
Ethiopia-may return, will they go for someone else other than Haile?
Huns-Scythia has basically taken their place, so return seems unlikely
Inca-definitely will return, possibly with Wayna Capac
Indonesia-may not return this time, wasn't a popular civ to play in Civ6, but I will definitely miss them

, maybe Philippines will get in over them
Iroquois-may return, or be replaced by another Native American civ
Korea-definite return, with Seondeok
Maya-definite return, with female leader, I hope

, Aztecs stole their ball court though
Mongolia-definite return, also would shock me if it didn't, I can't see them putting Kublai Khan over Genghis though
Morocco-may return, Islamic North Africa might return someday
Netherlands-definite return, expect Willem van Oranje again, or maybe Wilhelmina
Ottomans-definite return, possibly with Mehmed the Conqueror
Persia-definite return, would rather have a Sassanid leader this time, Shah Abbas is also cool
Polynesia-not sure if they will return, maybe the Maori will be in the game as a spiritual successor?
Portugal-definite return, hopefully not with Maria the Mad
Shoshone-probably will not return this time
Siam-may not return this time, to be replaced by Khmers, Vietnam, or Myanmar (all three are worthy of being in Civ in my humble opinion

)
Songhai-might be replaced by Mali this time, Saharan Africa is worthy of representation in Civ, even if they have Kongo
Sweden-likely will return, without Gustavus, is having three Scandinavians in the game overkill?
Venice-not sure about them returning
Zulu-they did returned for Civ5, so I don't think Firaxis will abandon them just yet