There would also have to be a unit that becomes the SMG troopers or light machine gun troopers. Ships and airplanes might also be considered, but I don't see airplanes happening for a mod in this game.
Starting from this game's Industrial Age.
Economy and technological advancements occur during this age.
Military begins to modernize weaponry. (The WW1 Age, where horses might still be used in battle.)
Nations begin to grow vast enough to conquer others. (The WW2 Age, where horses can no longer be used in combat--horse unit icons turn red/unavailble and new automobile unit icons appear or replace the horse unit ones.)
The atomic bomb can be researched in this era (like the titan of AOM, can be used to demolish an entire city). Getting here would obviously be costly and atomic bombs could only be used once per game...
The game starts with musketeers in the industrial age, but can upgrade to riflemen during that same age. Riflemen will eventually become "Repeater Riflemen" during the Modern Age. Each Rifleman upgrade will add a slightly higher rate of fire to the Musketeer/Rifleman class.
The game starts with Skirmishers in the industrial age, who have long range rifles. They can be upgraded to rangers in the Modern Age, giving Skirmishers an increased range and attack, but decreased rate of fire. Then they can be upgraded to snipers, the longest ranged infantry in the game, who, like the rangers, will gain increased range and attack, but decreased rate of fire.
Automobile upgrades come during the Imperial Age.
This could replace one of the artillery units or other ranged units. The Gatling/Machine gunners fire walls of bullets--best for taking out infantry groups.
The Anti-Infantry Cannon of the Industrial Age can be upgraded to tanks for the Modern and Imperial Ages.
Anti-Cannon Cannons become Anti-Tank Cannons.
The Anti-Build batteries.
Anyway, these are just some ideas for a mod.