One big reason I suspect (and still hope) for a third expansion to Civ VI is the list of who's missing from the list of Leaders from previous Civ games. If Gathering Storm does indeed end up being the final expansion for Civ VI, then Civ VI will be the first game in the franchise (to my knowledge; I started playing with Civ IV) to not include the likes of Napoleon and Bismarck, among probably others. I mean sure, there've been other French leaders besides Napoleon, other Japanese leaders besides Nobunaga, Roman leaders besides Julius or Augustus Caesar, and on down the line for each Civ, but you'd have to admit these are enormous omissions both historically and from a franchise perspective (on the flip side, let's not even mention the questionable historical accuracy of figures like Gilgamesh and Kupe). That's a lot to sacrifice to satisfy a checklist of Leader Agendas and (as has been suspected by some folks here on the forums) a gender representation quota. A lot of the Leaders in Civ VI simply lack the familiarity or the charisma of the old standbys which have become iconic to both their in-game and real-life civilizations.
I mentioned this in another thread, but when you examine the list of who's still missing from Civ IV and V alone...Napoleon, Nebuchadnezzar, Nobunaga, Kamehameha, Washington/Lincoln, Boudicca, Julius/Augustus Caesar, Bismarck, Pacal, Hiawatha, Wu Zetian, Pocatello (or any other Native American tribe from the Continental United States), etc....one would have to surmise there may well be a heavy militaristic theme to any expansion which would add them. And just as Eleanor brought a new concept...one Leader for multiple Civs...to Gathering Storm, might at least some of these offer a hint at what such a possible expansion's hook might be?
Consider Lincoln's role in American history (opposite Jefferson Davis). Or Nobunaga's efforts toward the military unification of Japan. Or "Roman against Roman" during the conquests of Julius Caesar. The countless conflicts of Dynastic China. Could the third expansion for Civilization VI...if it comes to pass...introduce civil wars and/or the concept of establishing dominance or unification of a specific Civilization between multiple Leaders before one goes on to represent and govern that Civilization on the global stage? Even if this would be a more Scenario-based mechanic, it would be intriguing to say the least.
At any rate, I guess time will tell regarding all of this speculation. I for one would eagerly support additional expansions and content for Civ VI as long as they add something meaningful (oh, and while I'm on the subject, I really hope Firaxis/2K/Aspyr/Take-Two will continue to support Civ VI on the Switch as well; at the very least they should communicate their long-term intentions to those who took a chance on the Switch version).