Don't forget the Easter Eggs! They're the spice of the game!

One idea I was wondering about was the use of heroes. These would be advanced vet units that you get a random chance at acquiring.

Requirements:

1) Your units have to be hard core vets before the random chance is available.

2) You can only build one hero per avalable hero type once it becomes available.

3) More than one hero type is available during the game for build

For instance, each Civ gets 3 or 4 types of hero units, but there is no guarantee that you get them. For example, you get armor, infantry, fighter, and phalanx units that have the ability to become heros. You would only be able to build 1 hero armor, 1 hero fighter, 1 hero phalanx, and 1 hero infantry. You could build all 4 of these units, but you can't build 6 hero fighters, 12 hero phalanxes, etc. 1 is the limit!

Hero units carry a special advantage, such as an extra attack strength or extra defense strength. Or, even BETTER, (but more work for the programmers) would be to give them some kind of special ability no other unit has.
They also get a special icon. (So you can show it off, and other players wonder what the hell it is when the game is first out).

Examples:

1. The French could get a "Joan of Arc" hero Knights unit, it would carry an extra attack factor with any unit it's stacked with, and increases happiness in any city in which it is garrisoned.

2. Germans get a special "Rommel" hero armor unit that allows for a stronger attacks and a "Blitzkrieg" ability that allows them to attack more often than normal armor.

3. US gets a "Big Red One" Infantry Special hero unit gets extra hit points, and extra movement points.

I could go on and on...

Like "A young maid Joan has inspired the rabble and wants to serve the country against her enemies" or something..

Then the hero could come with an army.. like 5 legions or something..

What I would emphasize with Heroes is randomness. You want the player's reaction to be, "cool! I got the Joan of Arc," right when he leasts expect it.

Heroes aren't supposed to be like common units. They're super rare characters that are hard to get. They're so rare that you only see them like every 20 games or so.

The ultimate hidden secrets in the game.

When you earn the hero character, your reaction should, be "whoaahhhh that was cool."

You remember the rumor of the megalopolis in Civ2? Where supposedly you put 4 cities together, and eventually it would form a real megalopolis? It's stuff like that that I think would be cool to discover.

I agree 100%!

"and only during the hard times"

That's an interesting take. Giving the player hope, a slight extra advantage, when he's down.

"this unit should also have Unique abilities and demand no support."