Hi,I think before we talk about the balance of the unit, we need to talk about the resources in the game.
In early game we need less oil, but a lot of goods, iron and rare material for research. In mid game the oil and grain is getting more important, the three other resources stays on high level. In late game we only looking for oil and grain.
So what about menpower? This is very different form nation to nation and from map to map, Sometimes you looking the whole game for men, if you have this problem you build barracks in every good province you find. Yes right, you only can do it if you have enough grain.
So that's why the grain and the menpower is the biggest problem for the infantry. They need a them both. In production and upkeep. That's the reason most players build only a huge amount of light tanks and arty as ground units. Arty you need to blow out the bunkers. Light tanks to flank the other troops an to take the provinces fast under cover of a large air force.
So if you find a enemy with a good air force you build anti air. If your enemy has a lot of tanks, you build anti tank to stop them. This both units we can build up cheap and fast.
Now we not build infantry, because you need the grain to feed you inhabitants in your growing nation and the menpower for all other units. Infantry was the main power during the WW2, but not in this game.
Suggestions:
The ships are all good for me, except the destroyers. They are a bit to weak against subs in my eyes. The only think that they are good for in this game, is to protect from subs and die as cannon fodder for the bigger ships. With subs you can attack with low cost and effective all other vessels or go spy the enemies coastline.
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In the air I think we need only to upper the range of the marine bombers, it's to boring that they have shorter range than the interceptors have, very often the distance to the next province center is greater than the range of the MBs.
We start building airbase for the fighters and after some days we finished the first MBs and we see that they can't flight the same route
Tactical Bombers are power full but to cheap in upkeep - look, a heavy tank coast double in oil than a TB! - give TBs 200 oil for upkeep to make land units more attractive.
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Give the regular infantry a bonus +50% in cities an +25% in Woods/Hills.
Give militia a bonus +25% in mountains and lower they upkeep. In this way this unit stays slow but will be best to protect the border form ambush. (especially when we become paratroops in game - yes yes yes we want them!)
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That's the major points to say for balance in my opinion. All other thinks we need to look in future after a new change...

