First off, Wolfshanze, I would like to give you great thanks for your mod! I enjoy it immensly and it adds a LOT of flavor to the game.
I was thinking about your upcoming addition of motorized infantry to the mod and I have a few thoughts on that:
Motorized infantry are WWII units just like tanks and marines, so they should be available with WWII techs - Industrialism, Radio, and Combustion. But to keep the computer (and human players) from using it to guard cities, remove the city garrison promotion from its availability on that unit. This would require a changing of its AI unit use from CITY_DEFENSE to CITY_ATTACK (or something similar). The unit would have

2,

20, and +25% vs. gunpowder units just like regular infantry.
Also, allow infantry to be upgraded to motorized infantry, but still allow infantry to be built at the same time. This way you have infantry in two roles - normal city defending units (available at WWI techs but still available in WWII era) and motorized infantry for use in the field. You could also upgrade regular infantry to motorized infantry for field use, or leave it alone for city garrison. Increase the cost of motorized infantry to 300

, and require oil to build.
This way you can have infantry keep up with tanks in the field.
Something else that has always bothered me is the lack of WWII mobile artillery. You don't get artillery that can keep up with tanks until you get the Laser technology, which occurs
much later in the tech tree. But this is not historically accurate. I suggest a similar addition like that of motorized infantry - motorized artillery.
It would also be available with WWII era techs. It would cost more than regular artillery (320

instead of 300

), but have less strength, say

16. Give it

2 and perhaps an additional bonus, such as +25% vs. gunpowder, to make it competitive against regular artillery in a field support role. You could also upgrade existing artillery to motorized artillery or still build regular artillery during WWII times.
Now with those two unit additions, you could have an all

2 WWII era complete blitzkrieg force of tanks, motorized infantry and motorized artillery.
NPM