If this is done correctly this would be a great way to not only expand the game, as well as a great way to define a unique playing style for each civ. Obviously some civs should dominate particular eras. When playing a civ like Egypt, one would rely on dominating one era. Likewise, Japan might have its unique units building and wonder spaced over a number of ages, therefore they are less powerful at any give time relative to other civs, but maintain some militaristic advantage over a longer period of time. Some civs (the mongols) might have two units of a similar type (mounted) and Germany would have a melee unit (Lansknecht) and of course their panzer tanks.
Here are my ideas as of now:
1. bring back the units from the unmodded game that were left out of HR.
2. Viking Longboats
3. either a bomber (B17) or minuteman for America
4. Katyusha rocket systems were the most feared weapon in the Soviet arsenal in WWII, they would make an excellent mobile artillery replacement for Russia
5. Roman Ballistas instead of catapults
6. Korean Turtleships
7. a Vietcong unit seems like a go for Vietnam
8. I like a cruiser for Japan. The Japanese were a naval superpower for the first half (almost) of the 20th century, this would be well represented by a cruiser. The Zero was obviously a good plane, but I do not think it was so much better than any other contemporary fighter that it should be a UU.
9. Israeli IDF fighter. Air superiority (particularly in the jet age) is a primary reason for Israel's survival.
10. French knights are a must
more to follow when i think of them
You have to remember that it is just another UU, it still won't change the whole strategy of a game.
But many more things like that can do so.
I also wonder what players' thoughts are on hypothetical extension of UUs? Meaning that what would an ancient culture produce if let to develop to modern or futuristic times. Is that something to look into or just leave out of the equation? For example, what would a Incan Rifelman be like; or a Australian Aborigine gunships; how about Sioux battleship?
Part of the game is theoretical and hyper-extending old cultures into modern ones, but having advanced theoretical units can also cause a lot of problems and issues.
Thoughts?
You have a suitable art for every unit in every civ, or every group of civs.
If you want, you can change the whole concept of UU to something different, and much more important.
For example - all Native American archery units have bonuses against gunpowder units.
All of the English naval units have specific bonuses.
All Arab units has something related to desert.
Russian melee and gunpowder units has defensive bonuses in snow.
Spanish units from the colonial age (Muskets, Cuirassiers/Conquistadors, Grenadiers) have greater bonuses when attacking less advanced units.
Assyrian melee units has city attack bonuses, or they can be less affected by walls.
I believe it is better than the battering ram, because than their good siege ability can be used in the ancient times.
Phoenician elephants has better movement.
No matter how many elephant units you are willing to add to the game, all of them will have the movement bonus.
These idea are inspired by the civ5 special abilities, but I want to make it much more important than in civ5, and I think that this way you can really have a full style of play for every civilization.