Let's talk about the picture: the buildings and units we can see
Buildings
Wonders - Kremlin
Artillery cannons - replaces cannon (base defence)
Sniper tower - replaces tower (base defence)
Anti-air gun - ?new base defence or ?replaces redoubt
Town centre - looks more modern
House - looks more modern
Large oil refinery
Small oil refinery
Oil derrick
Units
Airship
Biplane
Armoured car
Light Tank
Mobile artillery - ?replaces cannon
Ambulance - ?replaces supply train
Riflemen - ?replaces musketeer
?Anti-tank
Like CoC and the parent game Rise of Nations, I wouldn't be surprised if nexon added a 3rd resource OIL. This should balance out the ridiculously high cost of enlightenment gold and food costs, and should make it more worthwhile for industrial players to attack OTHER industrial players, instead of farming off players 1-3 ages below with no penalty (as it currently stands).
Let's discuss potential balance.
Tanks, planes etc should require OIL AND FOOD, to make their investment high. This should prevent players from spamming them.
Towers (of all ages) should be able to shoot at aircraft - this will give enlightenment players and below a chance to defend against them
Airships should be slow and target defences - this should give players strategy to use them.
Biplanes should be fast and target enemy units and planes, weak damage to buildings
Tanks should be slow, have a range of 2. but be unable to shoot over walls. Low damage to walls and buildings but high health.
Artillery should function like other cannons, high damage, low health, slow and vulnerable
Anti-tank or rocket troops should be the ones taking down walls and buildings, but they have to get very close.
Rifilemen - like musketeers, anti-infantry
Armoured car - could replace raiders and function the same way. Rise of Nations made them good against infantry, but vulnerable to tanks. If they made tanks take increased damage from rocket infantry, armoured cars could have a place in the game.
Not shown, but perhaps have a separate building for building planes e.g. airstrip, or tanks e.g. factory
Perhaps have a dedicated air defence building to defend against the introduction of bombers in the modern age (age after industrial) that cannot target ground units
thoughts?