While I would love to have the historical accuracy, I agree with BHG's decision to keep unique units for each country the same through all ages. Game balance is hard enough as it is with the different nations. Also, I can just imagine all the complaining about BHG "forcing" people to change...
Obviously pulling from Clash here, but seems like it would be a win/win to be able to use 20 crowns or something to boost barracks production rate by 4x for an hour. Players would LOVE the ability to more cheaply speed up training, and so would spend (and also buy) crowns to do so (thus profit...
Will we see the printing press have an impact in this era? I expect it to be researchable at the library and have an impact on economy somehow. Light bulbs/electricity also seem like a natural technology to implement, as do trains and trench warfare/creeping barrage - for this era.
Will there ever be any changes or additions to the trade goods? I can't imagine being able to hire industrial age mercenaries (or later modern/space age ones) with furs and leather...