I have a few little things that I think could be adjusted. To flesh out my comments earlier regarding the transition to the industrial era, I came up with two possible small changes, plus some thoughts on the modern age. Sorry that it's long winded.
The first is to permit steam power without requiring chemistry. There isn't a solid tie in the real world (you don't need chemistry to know that heated water boils, for example), and keeps a few options available to the player rather than jumping through the hoops for those chained techs.
That change alone may be enough, but I am also interested in a change to military science, possibly removing it as a prereq to chemistry. I'm not sold on it from a gameplay or real world standpoint but would take input (possibly make it follow military tradition or something along that train.
On to a bigger problem in the modern era, which is that the Motorized Infantry stats don't fit the role. It shows up too weak and too late in comparison to other units.
The Motorized infantry is on par with armor, but isn't available until people already have Heavy Tank, SAM and AT and are close to Paratroopers and Marines. The only advantage is the two movement, but the Heavy Tank is actually a better build - the MI only has a slight advantage fortified in a city on a hill (assuming the city defenses have been lowered).
After that the Mechanized Infantry appears far too late to be viable; the Main Battle Tank is in full force by that point.
I actually think that having near total domination by Heavy Tanks for a period of time is historically reasonable, but the MI should be beefed up to start providing a reasonable match by the time it appears - and better than the other build options.
I'm not good at balance, but here's a first shot...
Recommendation:
Motorized Infantry strength increased to 22 and cost lowered to 225, and movement reduced to 1.
This gives it a slight advantage against an unpromoted Heavy Tank when defending a city, disadvantage in the field, and leaves the player with the choice of fast moving tanks or a slower stack with additional defense options.
Change the order of techs in the modern/future era to:
Computer -> Robotics -> Quantum Mechanics -> AI
(move the Air Cav and Lab to Computer)
This moves the Mech Infantry up close enough to show up against the Main Battle Tank, moves the free tech plus cottage bonuses back, and makes more real world sense. The problems to be resolved are QM (no unit or building benefits) - maybe leave the lab on QM.
Machine Gun (195
18
1
+50% vs mounted and gunpowder, immune to collateral
Squad Infantry (195
18:strength 1
+25% vs mounted and gunpowder
Anti-Tank (225
18
1
+75% vs armored
SAM Infantry (225
18
1
+75% vs helicopters, +50% vs air
Motorized Infantry (240
20
2
+20% city defense, +20% vs gunpowder
Paratrooper (160
24:strength 1
can paradrop 7)
Marine (240
24
1
+50% attack vs machine gun/artillery, amphib
Heavy Tank (270
26
2
)