With Greece and India, I think the goal with multiple leaders is to represent multiple, separate entities within a civilization. Whether these entities existed simultaneously as rivals (Athens and Sparta), or in vastly different time periods, representing different cultures, languages, and religions, whilst still existing under the umbrella of a broader civilization (India). This gives us more representation per civ than any previous Civ game, as we can, for example, represent all the different cultures of India without balkanizing the country (pending future leaders), an act that would've been highly unlikely in this Eurocentric franchise before now.
Now, India has a very broad design to it, which appeals to vast swaths of Indian history, but other civs, like England, are heavily focused on a very specific period in time (in this case Victorian England). Therefore, we got medieval Scotland instead of a new English leader.
The first objective, I believe, is that these leaders have their own capitals. Secondly, and more importantly, they should represent different realms. It's weird enough to have Romans and Americans and Kongolese going at it, but for France to divide itself between two monarchs of the same dynasty? Hm. Unless we change it up and make the two leaders mutually exclusive, as in Civ II and IV, I don't see it happening. And as we've seen, that's not how it works in Civ VI.
For these reasons, as awesome as it'd be to have a French leader like Philip Augustus or Charles Martel, it's unlikely as they both shared a capital city with Catherine: Paris. And besides, France has historically only been one realm, and any other "France" (Middle and East Francia) were more German than French. Any earlier than that and we get the Gauls, who hardly fit in with their very wonder-focused theme. Honestly, the only leader I can see them going with (if any) is Jeanne d'Arc, leading from Orleans, as not being a monarch and therefore independent of the French throne would work in her favour here. Though it would still be kind of awkward.
Finally, these leaders should employ a dichotomy: the warrior-queen Gorgo gains culture from kills, whilst Pericles gains culture from city-states. Gandhi is all about eating dandelions while Chandi punches his teeth in. I can see something similar with the more defensive Teddy Roosevelt and internationalist Harry Truman, but America has only ever been one country (except for that one time, but they'll never go for it, because 2018).
Anyways. Since they were first revealed, there were a few civs that jumped out at me as peculiar, mainly because they had different capitals than they've had in every other Civ game, though that is but one reason why they are absolutely ideal candidates for multiple leaders:
1. Germany. As Barbarossa represents the medieval Holy Roman Empire from Aachen, a Prussian king could step up to lead the German people from Berlin. Both entities existed simultaneously, both existed within the umbrella of the German civilization, and a dichotomy between them exists in the form of the Protestant/Catholic schism.
2. Egypt. Cleopatra the Greek queen of Alexandria vs. a real Egyptian king of Memphis or Thebes (or Akhetaten... please let it be Akhetaten).
3. China. A militaristic Manchu Qing usurper leading from Beijing would stand in perfect contrast to the native Chinese wonder-building *false* emperor in Xian.
4. Russia. Before Russia there was Muscovy, ruled by strong men of the Cossack line. True bearded Russian princes who defended their "backwards culture" (Peter's words not mine) from invaders from the east before Peter cut their beards and sold their traditions out to the west. The Ivans probably want their revenge.
5. India. While the dichotomy is already explored between Chandi and Gandhi, there could still be room in India for the Mughals. The middle of two extremes, as it were. Though it's also possible they could pull a Macedon on this one, since, like the Macedonians, the Mughals were outsiders who weren't really considered "Indian" until they changed what "Indian" meant (and boy did they!). Other cultures could also be represented.
There are surely others that "qualify" as well, but these are just the ones that stuck out to me. And frankly, I'm not banking on being completely right on this, but I'm feeling pretty good about it.
Aztec, Mongolia, Zulu, Cree, Mapuche, Kongo, Nubia, Scotland, Scythia, Sumer, Brazil, Australia. Sorry.
it could get Mughal ruler to represent Muslims and gunpowder tolerant empires, or Chola ruler to get maritime bonuses, or many others.
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