My opinion is-
Japan - Zero -
The Zero should be about equivelent with other fighters but cheaper and with an increased range. This is because the Zero did have a good range but didn't have an increased ability to fight. Its menuevering was on par with the U.S.'s Typhoon which wasn't all that great. The Hellcat later on ate Zero's for breakfast. However in my opinion the greatest advantage the Japanese ever had was cheaper aircraft carriers with a larger capacity for air. How great of an advantage this would be I don't know.
Russia - Mobile Scud
The Mobile Scud should be very cheap to produce and it is discovered some time before the cruise missle (I suggest beginning of Modern or better end of industrial). It should be as powerful as the cruise missle but with a cheaper cost and 2 moves instead of one, due to the fact it fires from a vehicle. Once fired its gone. This was one of the more significant signs of power during the peak (to this date) of Russian authority. Though the T-72 was their ground power it was on par with the M60, and once the patton series went into production the US had a better tank.
England - Royal Infantry
Though the Royal Navy was the most significent weapon at Englands peak, its a bad unit. The quality of Englands troops during the 1700-1800's was magnificent though. They were some of the more dedicated and well trained troops in all the world. Sailing htem around the world just wasn't her majesties best idea though (hey i don't mind, im American). These infantry would be based around the time of muskets with an increased defense so they can be employed when the rifle is discovered as well. England has never been invaded since the Normans and though that was a testement to the power of her Navy, you certienly wouldn't of wanted to deal with her armies once on the island as well.
France - Musketman+
The Musketman should be upgraded to be able to counter the Royal Infantry. Theres no doubt that France and England were the two greatest powers of the 1700's and 1800's. Their peak was reached at seperate times but both were always able to counter one another. The Musketman should have an increased attack.
America - Abrahm Tank
Though our airpower is significant it isn't increasingly advanced in ability, with the exception of stealth. However our M1a1 Abrahm is undoubtably the most powerful peice of ground equipment paying no mind to nuclear weapons. It isn't more powerful in firepower but it can take a greater amount of hits from Sabot rounds (it an anti armor tank round) then its only competition on the battlefield the T-80. My own personal opinion is that precision weapons are the U.S.'s greatest weapon but I dont know how you could get a cruise missle type of object to be able to destroy buildings or units with a heavily increased rate of accuracy. If you could get that to work that would be excellent but if not I think the M1a1 Abrahm is your best bet.
Every unit has been decided off of the pinacle of each nations power. England and France were both their most powerful during the 1600 - 1800's which is why they have their units. Japan was at her height during 1935 - 1945. Russia was at her peak during the late 1950's - late 1970's. And America is more powerful then it has ever been. I think all nations should have a unit that is based on their period of greatest influence and power over their history. This doesn't mean they had to be powerful, just at their peak of power.
Imagine an automatic rifle in 250BC except there is no bullet and gunpowder but bolts. This has to be the UU of the chinese
