some thoughts-
how about adding region-specific horse types to the units? for example, the britons get crappy, low-hp horses(as I understand it, the crappiness of their horses is what led them to use chariots), whereas steppe peoples get a light(archer) and heavy(armored) horse. Slightly lower charges, higher speed and maneuverability than native, higher hp(to simulate the vast endurance advantage). Nabateans, nubian cavalry and high tier ptolemaic/seleucids can have arabian horses(extremely high quality breed even today)
Europe- I don't think gallic cavalry was anything to write home about. I see plenty of mention of them used as auxiliaries, but never anything about them making any decisive difference.
Germans, however, are mentioned by caesar as having very high quality horsemen, and I believe german cavalry were often used as personal bodyguards. the colder climate would favor larger horses.
italy- roman cavalry were the redheaded stepchildren of the ancient world. Give them crappy horses. If roman players want good cavalry, they can hire mercenaries, just like the real ones did.
Spain- we associate modern spain with good horses, but does this predate the moorish introduction of the arabian? beats me.
as for the greeks, I assume horses of any decent quality were expensive and rare, seeing as the territory is unsuitable and even lords fought on foot.
macedon- obviously phillip and alexander had good melee cavalry, although I'm not sure where their horses came from. I'd assume from well abroad