stealth_nsk
Deity
The more info we have about modern civs, the more probable it looks for one of the future expansions to add 4th era. All modern civs announced so far ceased to exist in 1870-1950 period and names like Qing China or Meiji Japan suppose there is more modern (contemporary?) age for those civs. The choice of Buganda as modern civ also implies Firaxis wanted modern age civs which didn't survive by now.
For not yet announced civs, the harder one to fit into this concept is, probably USA as it went the least changed from its independence period. The rest looks quite clear:
British Empire (modern) -> Great Britain (contemporary)
Prussia (modern) -> Germany (contemporary)
Russian Empire (modern) -> Russian Federation (contemporary)
etc.
For not yet announced civs, the harder one to fit into this concept is, probably USA as it went the least changed from its independence period. The rest looks quite clear:
British Empire (modern) -> Great Britain (contemporary)
Prussia (modern) -> Germany (contemporary)
Russian Empire (modern) -> Russian Federation (contemporary)
etc.