How about increasing the strength of the Musketman to 10 and decreasing that of Infantry to 18? Lastly add a new unit: Modern Infantry / Squad Infantry with strength 24, +25% City Combat available at Plastics. It never made much sense to me that after Robotics you can suddenly draft mechanized vehicles.
Also can we please please please nerf Axes or revamp them entirely? How does it make sense that a bunch of thugs armed with axes can consistently wipe out an orderly formation armed with spears, which is what Spearmen represent? It should be the other way around. Axes should be good in forests and cities, spears on open fields and hills. Also Horse Archers don't work as they really did either. Historically they could run literal circles around infantry formations (yes with spears), harassing them with arrows and galloping away before the enemy could so much as scratch them. The best way to deal with them was with archers, which ironically are about the worst unit to use in the game for that purpose.
Proposal for new classical unit balance:
Axeman: 30 Hammers (instead of 35), 4 Strength, 1 Move, +25% Forest, +25% City Strength, requires Copper or Iron
Spearman: 35 Hammers, 4 Strength, 1 Move, +25% vs. Melee, +50% vs. Mounted
Archer: 25 Hammers, 3 Strength, 1 Move, +25% Hills Defense, +50% City Defense, +50% vs. Mounted
Also remove Immunity against first strikes of Horse Archers, heck maybe all mounted units except maybe uniques, they have easy access to Flanking 2 which provides it anyway so it's redundant.
Edit:
Oh and Pikes and Heavy Footmen! Give the former +25% vs. Melee and +50% vs. Mounted and half the latter's bonus vs. Melee to 25% instead of 50.