Well if we look at things missing from past games as a precedent (which we shouldn't really but I did the data so oh well)
Civilizations
5 Appearances - Babylon
3 Appearances - Byzantium, The Maya, Portugal
2 Appearances - Ethiopia, Iroquois
1 Appearance - Assyria, Austria, Hittite, Huns, Morocco, Shoshone, Siam, Songhai, Sioux, Venice
(excluding ones that have a version in this game ie Celts/Scotland or The Vikings/Norway/Sweden)
World Wonders
5 Appearances - Globe Theatre#, Great Wall*, United Nations#,
3 Appearances - Hoover Dam, Kremlin*, Pentagon#, Statue of Zeus,
2 Appearances - Notre Dame, Parthenon*,
1 Appearance - Apostolic Palace, Borobudur, Brandenburg Gate, CN Tower, Great Mosque of Djenne, Himeji Castle, Leaning Tower of Pisa, Louvre, Neuschwanstein, Porcelain Tower, Red Fort, Shwedagon Paya, Spiral Minaret, Three Gorges Dam, Uffizi, Versailles, Hubble Space Telescope
(* means that it appears in civ 6 in some form, # means that the number of appearances induces it appearing in other forms than just world wonder)
Natural Wonders
1 Appearance - Barringer Crater, Cerro de Potosi, Grand Mesa, Krakatoa*, Lake Victoria*, Mt Fuji*, Mt Kailash, Mt Sinai*, Old Faithful, Rock of Gibraltar, Sri Pada
(* means they are in Civ 6 as a
Geographical Feature)
Analysis
When it comes to Civilisations the fact that Babylon is not in the game as a full civ, is probably the best clue that there is something else coming, or that they are ignoring years of precedent (and some hints in the civiliopedia). You would expect also that many of the other multiple re-occurrence civs like The Maya would also be considered MIA if there is nothing else coming.
For the World Wonders, it gets a bit more tricky because they have a lot more one off appearances than any other part in the game. However, given that we have all the other 7 Wonders of the World (Pyramids, Colossus, Hanging Gardens, Great Lighthouse, Mausoleum at Halicarnassus & the Temple of Artemis) as well as all the New 7 Wonders of the World (Great Wall*, Petra, Cristo Redentor, Machu Picchu, Chichen itza, Colossum & Taj Mahal) then the lack of Temple of Zeus is telling. And you would think that they would add Notre Dame as well.
For the Natural Wonders, this is a more fluid list as this is only the second time that they have been in the game, but you would think that there are several more examples that they could add to the game if they wanted to.