I just want to add, that is this idea is to be worked on we should also aim towards realism.
Yes, mercenaries were used, what I mean by realisim is about the civilizations and units in question:
Rome, bad idea, in the time of Grepolis Rome did not even exist as a republic yet, it was a small kingdom which uses greek and etruscan kind of units, the used hoplites, they were weak and had no trade power. A Roman ship is unrealistic, let alone the units it offers, Roman Cavalry? the romans had awful cavalry, their armies had very few Equites which were poorly supplied for horsemenship. So both Rome and its units should be scratched from the list.
Also thought I should address this:
I am probably overthinking this but why is the Roman Legionary attack sharp when all the legionaries used javelins as ranged weapons then switched to short swords during close combat. The swordsman which would be the regular grepolis counterpart is a blunt attacker then wouldn't the Roman Legionary have a blunt attack to.
Roman Legions were "sharp" not blunt, firts because they threw their sharp Pila before engaging in hand to hand combat, with both the "spatha" and then the "gladius" were sharp swords used for thrusting, not slashing or cutting, they were thrusted to the enemy in a punch like movement, Roman swords were not blunt weapons, if soldiers tried to hit with them in slashing movements the swords would have broken.
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but anyway Romans do not fit in the historical setting of the game.
Gauls: While they did exist at that time, they were not yet used as mercenaries and kept to themselves in their forests, if you want "barbaric" like mercenaries, switch to Tracians instead, they were around the time, and they were used as mercenaries in many occasions, the most famous were the tracians Alexander took to conquer Persia.
Units: barbarian skirmisher, barbarian cavalry.
The tracians were noted for their horses, in fact the Macedonians learned the use of horses in war because of their proximity with the tracians.
As for cartage, well the units are ok, but you change them for Numidian Cavalry (they had javelins) and Baleares Slingers (from the Baleares islands) cartaginian armies were famous by both units, although the numidians were mercenaries themselves, from Numidia, we can have Carthage selling them, for their proximity with Numidia