I'm pretty sure you can calculate, mathematically how many leaders we'll get.
Every leader has two possible Civs per era and each Civ appears to have two possible default leaders.
So 10 Civs per Era => 20 leaders in base? And each leader has 6 historical choices in total (2 per era).
I'm poor at maths, so I'm not sure if this works out, but looking at it with strictly the Antiquity Civs (without the additional personas)
Egypt: Hatshepsut or Amina
Aksum: Hatsepsut or Amina
Maurya: Ashoka or (??)
Khmer: Ashoka or Trung Trac
Han: Confucius or (??) (Himiko?)
Persia: Xerxes or (??)
Mississipians: ?? or ?? (+Tecumseh in DLC)
Maya: ?? or ??
Rome: Augustus or ?? (+Napoleon in DLC)
Greece: Machiavelli or Franklin
For each leader we know they have the following historical choices:
Augustus: Rome, Spain, French Empire
Hatshepsut: Aksum, Egypt, Abbasid, Buganda
Amina: Aksum, Egypt, Songhai, Buganda
Franklin: Rome, Greece, Norman, (United States)
Macchiavelli: Greece, Norman
Napoleon: Rome, Norman, French Empire
Tecumseh: Mississipian, Shawnee
Ashoka: Maurya, Khmer, Chola, Mughal
Confucius: Han, (Ming), (Qing)
Trung Trac: Khmer
Himiko: Majapahit, Meiji
Xerxes: Persia, Abbasid
so from this I would infer the following information:
(1) No historical choices for Mongolia and Maya yet, meaning that those leaders are yet to follow.
(2) All Euro leaders so far are male. One of the, Napoleon is missable free DLC. Another, Franklin, arguably doesn't even count as he's the American leader. I think we're due one more Euro leader and whoever it is, they're likely to be female
(3) Amina and Hatshepsut are the only leaders in Africa and Hatshepsut is tied to west-Asian Civs due to her Egyptian status. We could get another one there, and I would suspect this leader to be male.
(4) Napoleon and Franklin are the ONLY modern leader so far. This feels like a low number, and I would expect at least two more (maybe following the criteria above).
Right now we have twelve (including Napoleon and Tecumseh) and I don't believe we're getting many more. I would suspect at least three more, maybe four? An American leader of sorts (Monty, Bolivar, Pachacuti or someone else entirely), a female Euro leader (Isabella, Elizabeth, Victoria, Jeanne d'Arc, Marie Curie) and a leader for Mongolia (Genghis Khan, Tamerlane) would be my guesses for the three unrevealed leaders that I think we're 100% due.