It might be possible to apply a default, non-upgradable infantry promotion related to the country's war-readiness, production level, and quality of leadership. This way you can keep most infantry Strength stats generally consistent but allow for variations between countries. Note, I am
only talking about the Strength stat here and am paying no attention to cost, movement, other promotions, etc. I understand this is borderline to Dale's approach of minimizing promotions to avoid grognardiness, but if you keep it simple and tie it to things like barracks and production levels I don't think it'd be difficult for people to pick up.
I'm thinking of four basic levels:
With a few exceptions you could even restrict the promotion levels to troop types, so early infantry could only have L1 to L3, and Improved or later infantry could only have L2 to L4 because "improved" indicates a fundamental alteration in quality. You could of course restrict this by country as needed.
There are a lot of things you could do with this. You could tie it to civics (fascist industry or industry L3 and higher gains a +1 organization level), traits (Industrious or Organized provides a +1), or buildings (barracks provide a +1). You might be able to move the barracks to an early tech that some countries don't initially have, so you're limited to the initial quality of your troops until the research comes in.
You could even tie this to events. For example, when you hit Stalin's purge of the military of 1937, you could do something like reduce all newly built infantry by -1 for a certain period of time.
Here's a quick look at some numbers:
"Counteracting" the default organization type would come with experience. I am not familiar enough with promotions in Civ4 modding to know if you could simply upgrade them upon reaching a certain level (ie, if unit lvl 3 is reached and unit has L1 or L2 organizational promotion, replace original with L2 or L3 respectively; if unit lvl 5 is reached and unit has L2 organizational promotion, replace original with L3).
You can generally assume that the "unnamed" countries will start off with Poorly Organized (L2), and then apply basic categorical variations to the countries themselves to start them off with the numbers you want.
For example, in 1936 Germany could start at Organized (L3), and cities with barracks could produce Well Organized (L4) troops. The U.S. might start as Disorganized (L1) or Poorly Organized (L2) to represent the abysmal state of its army during that time period. Both Chinese factions could start as Disorganized (L1) or Poorly Organized (L2) to represent the devastating effects of its civil war which left if vulnerable to external forces. Etc, etc.
Anyway, that's where I'm at. Obviously this needs balancing and testing, but it's something the player can feel in control of through tech chasing and smart play rather than feeling stuck with "crappy units" through the course of the game.
isn't all that matters for making a tank useful, especially the way I play. I often do better with lighter, faster tanks. And that's the point, there's niches for different types of tanks at the same time; in some cases, a light fast tank will do better than a strong slow tank! Esp when there's aircraft involved, aircraft with long range, and you want someone to race in and seize the land you've softened up. That's why tanks aren't designated as "obsolete tanks" and "current tanks" but rather as "light" "medium" and "heavy" tanks, none of them obsolete until a new tank of the same class is developed, a new tank that has all the strengths of its predecessor with no new weaknesses. Heavy tanks do have all the oomph of light tanks and more, but they have weaknesses the light tanks don't have, like slowness. So yeah, I love variety of troops in wargames, so I can craft my army according to my strategy. If you're willing to put in the work, boozer and Dale, then I think a lot of us will like it 
, starts with amphibious, immune to damage from Nukes?
