If the game was going to be about the world wars I would start it off a bit earlier, around 1800 or so. This makes it seem more logical to start with a near empty tech tree, because neither rifles nor steam ships nor cars nor planes are really a thing at this point. You can just ignore the existence of sailing warships and horses and start with just musketeers as units. WW1 would be age 3 in this setup. This makes sense because age 3, if we’re staying relatively close to aoe2, is the one where all important unit lines exist and the real fighting gets on the way. Tanks (light and heavy even), fighters, bombers, battleships, they are all developed before or during this war. Age 4 is the bigger and better age. And aside from aircraft carriers (not sure if or at least how this should be a thing in the game, maybe just have planes as separate units), nukes (sort of an end game ruining weapon at best) and radar (a technology extending line of sight or such) this kind of holds true for the real military technology of the time. That would leave age 2 as roughly 1850-1914, rifles, armored cars, scout planes, ironclads, the works.
Not sure if I’d be hyped, but I can see this kind of game work. I just wouldn’t want it to be overly specific in which country had which exact units I think. Keep it aoe style. I also think that if you’re going to include Japan as a base civ you need to give them at least China to fight. Pearl Harbor and the late pacific theater don’t tell enough of a story to justify using the whole period.