I'm really not going to deny that this hasn't been said at least 1000 times, but
Australia (not just on account of having their geographical inclusion being brilliant on TSL maps) would be as super to have in the game.
Sure, it's a Colonial nation, a constitutional monarchy and historically a loose bunch of co-existent native tribes, but I find Australia has a lot to offer and seeing as both the Russian and German civs in this game borrow UA's, UB's and UU's from throughout their century-long existence as an empire/peoples that cover a general geographic area.
So I'm not asking for an Aboriginal civ per say, or even just a strictly modern one, but rather one in the same vein as the two aforementioned civs. Strictly speaking - no, Australia is not a Civilization - but it's unique history as an amazingly vast and isolated landmass with a multicultural peoples and a thoroughly rich and interesting history (even as a colonial nation) is at least worthy of an inclusion in the series.
I mean, off the top of your head one could easily come up a myriad of options pertaining to an Australian 'Civ':
Emblem: White Field - Ochre Aboriginal Waterhole symbol (inverse of England), Green field and Gold symbol works too.
Start Bias: Coastal, near Stone
Leader: Menzies/Barton/Chifley
Unique Ability: Upstart Convicts - Settlers are 50% cheaper and found cities that provide +5
Unhappiness for 20 turns. +3
Happiness for Coastal Cities
-or-
Unique Ability: Dreamtime - -50% off your next Golden Age for every natural Wonder Discovered (one per NW). +1
for every unworked stone tile
-or-
Unique Ability: Boundless Plains to Share - +2
Happiness for Coastal Cities. +1
Population for first 3 Coastal Cities every 10 turns once Australia reaches the Modern Age
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Unique Unit: Aboriginal Hunter - Has ranged attack. Unique Promotion (Stealth) Ignore enemy zone of control. Replaces Scout, Upgrades to Archer.
Unique Unit: Digger Infantry - Unique Promotion (Trench Warfare) 35% Fortification Defense Bonus, replaces Great War Infantry.
Pick 'n choose. There are heaps more options out there, but these are just the ones I scraped off the top of my head.