Just a thought, but considering the unique traits/units/interactions of each civ might help decide who should be in, since the more original and interactive (given real locations)
would be the most enjoyable to play. Some suggestions (and opinions):
Inca Expansionist/Industrious
UU: Slinger (3-1-1?) or a warrior/spearman variant based on the bronze mace favored by their warriors
RELATIONS: only with Spanish and late at that - but they stand also for Chimu, Tihuanaco, etc, etc
VERDICT: IN as they represent a whole culture region (plus the unique twist of playing Andes region - and doing without horses!)
Ethiopia/Axum Religious/Commercial
UU: would actually prefer an elephant here (at least one was used in Arabia in the 6th century) but no real standout here
REL: Egypt, Rome, Persia, Arabs even before 600 plus English (minor war) later
VERDICT: IN, too much history here and this region really needs at LEAST one civ to fill it
Mali/Songhai Commercial?/?
UU: ?
REL: Arabs, western colonial powers later?
VERDICT: MAYBE: would really like to add one more west african civ and one of these is probably the best choice, but they're largely isolated and would really need a good unit to add to "fun" factor of playing (and to justify leaving another one out)
Tibet Militaristic/Religious
UU: Cataphract - very heavy cavalry - (5-3-2?) which was the heart of Tibets armies during its medieval empire
REL: India, China, Turks?(pre Ottoman), Mongols.
VERDICT: MAYBE. Fun location and challenge in the mountainous region, but unique unit spinoff (from knight) is already crowded. They need Chinese to change to more realistic Commercial/Scientific and ANYTHING but the "RIDER" (WORST unique unit ever!!!) to be a really good alternative.
Poland Militaristic/Expansionist?
UU: Cavalry upgrade (parallel to Cossack), perhaps 7-3-2?
REL: Germany, Russia, Vikings(Sweden), Ottomans
VERDICT: IN? unless bumped for Hungary. Huge empire (with Lithuania) and lots of history. Also flexible after Militaristic as far as I can see (but with "heroism being the Polish vice" that one trait's locked in I think)
Hungary ?/?
UU: Horse Archer (4-2-3?)
REL: French. Poles, Ottomans, Germans
VERDICT: IN? unless bumped to keep just Poland. History back to middle ages and incessant raiding of western Europe by mobile horse archers would make for interesting play. Real "tweener" for me on civ type though.
Judea Commercial/Religious
UU: Zealot light infantry 3-2-2?(that fought Seleucids/Romans) or any modern one that fits (Merkava perhaps)
REL: Egypt, Babylon, Greeks(Seleucids), Romans, then later Arabs
VERDICT: IN?, though they crowd an already full region. Would really enjoy another modern unique unit though.
now either
Dutch Commercial/?
UU: possibly a frigate upgrade, more likely caravel
or
Portugese Commercial/?
UU: definitely caravel upgrade (2-2-3?)
VERDICT: one of these IN? Problem here for me is these civs don't stand out from one another. Dutch time of dominance followed close on Portugese time of dominance with similar "style" of empire. (in fact the Dutch occupied part of the Portugese one). Wouldn't take both for sure if Poland and Hungary would be left out - unless it was a 1492-ish mod of course.
Maya Religious/Scientific
UU: Warrior or Archer variation again?
REL: Aztecs somewhat, spanish
VERDICT: very MAYBE, great cuture but would crowd central american region badly. Unique unit might be repetitive as well, and couldn't be given stats that'd outdo Aztecs militarily (make Jaguar warrior a 2-1-1? or 2-1-2? as a fix?).
others I lack 'game ideas' for:
Macedon - (militaristic, commercial) - too tied up with Greeks as it is now and too like the Romans (civ traits, UU Phalangite 3-3-1?) for some. But I myself will include them, perhaps with a companion cavalry offshoot (3-2-2?) as well. (my user name is a 'Seleucid fan' giveaway I'm sure - of COURSE they're in MY mod!)
Polynesian - a very unique and challenging situation for sure, but very difficult for play balance. It'd be fun to found Easter Island and run an early maritime 'empire' though.
Srijava - (commercial, religious?) dominant in Indonesia for hundreds of years, controlled China-India trade route. No real idea on an unique unit, perhaps an eastern tpe of caravel?Definitely would fill a gap in terms of highly populated regions.
Khmer empire - similar to Srijava; Angkor Wat gives them a "fun factor" edge versus the equally worthy Thais or Vietnamese. Would fill a neglected but highly civilized/populated region, but I have no suggestions on a new unique unit here. (why do I always want to use elephants in these situations)
Sioux - would like to include them or other north american tribes, but I think the Iroquois stole their unit. I'd probably have to give them the mounted warrior and give the Iroquois a foot combat unit that treated all terrain as roads (archer 2-1-1?)
Anyway, a few ideas and examples to think about.