Because of the nature of this mod, quite a lot of the 'concept' art is already in the game. The advantage of this mod over the more Totally Converted mods is that it will be possible to build a reasonable looking working model of quite a few of the factions while only having to slightly scratch the surface of possible modification. A lot of the later units are already in the game (as early units).
This will allow the game mechanics to take priority at first with a vast host of other units to be added later for non-covered factions.

Some ideas though. Macedon should use this symbol:

The Argead Star, rather than the Greek letter in the vanilla game for two reasons. 1) Its far more accurate and better and they are the title faction 2) If you choose to include the Spartans then they should use the Lamba symbol and the two would look too similar.

Athenians: