Also, regarding the addition of elite and tank troops into the game, here's my thoughts:
Already-existing units would stay the way they are.
Elite troops would start off as something like Ranger Recruit, then upgrade to a Ranger Veteran. Elite troops would be better than regular troops, but to balance this, elites would also be much more expensive than regulars and would also take longer to train (to make it hurt more when you lose one).
Tanks and tank infantry:
First, you would have to recruit tank crewmen to your party. These would only be recruited at army bases (castles) and cities. The guys who made Gekokujo figured out a way to make it so that you could only recruit advanced soldiers from castles and cities, maybe see that mod for reference. The tank crewmen themselves would only be armed with only a pistol, but are also the only ones able to drive a tank. Then, you have to actually buy a tank. Referencing another mod, the people who made Twilight of the Sun King did a thing where upon visiting a city, you could visit a canon foundry and buy up to three(?) cannons for your party, which would be used in battle. Maybe in the same way, the player could visit a tank factory or something where you could buy tanks based on how many tank crewmen are in your party. Tanks would have a really high armor rating to the point of where they would be unaffected by bullets, but weak against grenades and AT rifles (and other tanks). They would move relatively slowly, but with a powerful cannon to use against enemy troops. Maybe to simulate being able to run over enemy soldiers, you could even give tanks a really high charge rating, allowing them to knock out enemy soldiers by driving over them.