Similar to the way the Monitor shoehorns the bombardment category of ships into the Industrial Age, I've been thinking of other units that could be included in this manner...

Heavy Tank
Industrial Age. Takes advantage of the vacated unit slot as a nation moves from cavalry to armored vehicles. Very slow, but very durable and packs a lot of firepower. Examples: British Mark V, German A7V, etc.

Tank Destroyer
Modern Age. Less durable than normal tanks and has little effect against infantry, but deals heavy damage to enemy tanks and has slightly better range. Examples: American Wolverine, German Hetzer, etc.

Mobile Gun System
Info Age. Closer-ranged and less armored than a normal tank, but is even faster than armored cavalry and can fire while moving. Strong against infantry, armored cavalry, and buildings. Examples: American M1128 Stryker, Russian Sprut-SD, etc.

Airship
Industrial Age. Instead of hulking, slow-moving siege bombers, I think a smaller coastal airship specifically made for hunting submarines and other ships would fit better as a pre-helicopter unit.