Emperor Xerxes: The Byzantines flags can be just like the Greeks but red. Also branching off this idea.. the Iroquois modern flag could possibly be the Canadian flag but blue rather than red.
This morning since I had Saturday School (4 hours of doing nothing) I decided to come up with a list for every civilization including the flag for Ancient, Middle, Industrial, and Modern ages. It's incomplete but I have completed the entire row for a few civs.. this is what I have for those certain ones.
America:
A- Snake flag
M- Colonial flag (13 stars) (made by SprincruS)
I- 1867-77 flag (37 stars)
M- Current flag (50 stars)
Austria:
A- Yellow/black/white flag
B- Red/white/red flag
C- Red/white/red / red/white/green flag (Austro-Hungary)
D- Red/white/red w/coat (made by SprincruS)
Byzantines:
A- Red flag with white couped cross
B- St, George's cross flag (made by SprincruS)
C- Paleoleague Orthodox cross on red flag
D- Byzantine double-headed eagle over red flag
China:
A- Imperial flag w/no dragon (made by SprincruS)
B- Imperial flag w/dragon (made by SprincruS)
C- Republic of China flag/Current Taiwan
D- People's Republic of China (made by SprincruS)
England:
A- St. George's Cross (made by SprincruS)
B- St. George's Cross w/coat (made by SprincruS)
C- British Commonwealth flag? (made by Firaxis)
D- Union Jack (made already?)
Germany:
A- Some sort of Germanic symbol, any ideas?
B- Teutonic Order flag (made by SprincruS)
C- Nazi Germany [naval] (made by SprincruS)
D- DDR of Germany (made by SprincruS)
Greece:
A- Shield symbol (made by SprincruS)
B- Blue flag with white cross
C- Flag w/coat (made by SprincruS)
D- Current flag (made by SprincruS?)
Rome:
A- SPQR flag (made by SprincruS)
B- HRE flag (made by SprincruS?)
C- Italy w/coat (made by SprincruS)
D - Current Italy
Vikings:
A- Some sort of symbol
B- Another symbol
C- Denmark
D- Norway
Well there's some for you guys. For a lot of these flags we will have to create by ourselves because flags weren't really used until the 1600's. The only people to really carry flags before then were the Scythians, Huns, and other Turkic tribes.. and those flags were rather differating and extremely simplistic with symbols of animals, stars, etc.