A commendable thread, and it's high time I posted my thoughts.
New Civs - Why not 10? We need to be on track for 50+ this time, and who knows how many more DLC there are.
1.
Babylonians (Hammurabi or Nebuchadnezzar) - a classic, made all the more necessary by Sumer's poor showing
2.
Incas (Pachacuti or another Sapa Inca) - South America is virtually empty, and they are the prime candidate
3.
Ottoman Turks (Suleiman or Roxelana

) - a well-known empire that lasted many centuries and has a sizable fanbase
4.
Mayans (Siyaj K'ak' [Fire is Born]) - arguably the most important Pre-Columbian civilization
5. a
Native American tribe - I've seen numerous suggestions that are quite feasible, including the Creek, the Haida and the Powhatan, though I think the Cherokee or Sioux are most likely
6.
Mongols (Kublai Khan for a change) - obvious reasons
7.
Koreans (Seondeok) - another highly important east Asian civilization, often an XP1 civ
8.
Carthaginians (Hannibal, just because!) - popular, usually seen in XP1, more war elephants needed
9.
Gauls (Vercingetorix) - a token European civ, since ancient Europe is underrepresented compared to medieval/modern Europe. The Dutch and Portuguese can wait their turn.
10.
Malians/Songhai (Mansa Musa/Askia Muhammad) - use the two leader mechanism to include both of these neighboring empires with considerable overlap
Second Leaders
1.
Ashoka (India) - because India is in desperate need of a non-Gandhi leader
2.
Khufu or
Akhenaten (Egypt) - a more ancient ruler for Egypt
3.
Ivan the Terrible (Russia) - an eclectic choice for Russia, just imagine having both agendas secret
4.
Alfred the Great (England) - the greatest English ruler, think of how cool it would be to have a leader speaking Old English
5.
Isabella (Spain) - just because we know she was in development and has been in the works for some time
Honorable Mention
6.
Louis XIV or
Cardinal Richelieu (France) - a French leader for France