How about spilting the promotion system. One set can be bought with for experience gained in combat, much in the same way it does now. The other set of promotions would be upgrades, and this could be done when building the unit. They would cost production, ie to take longer to build, this would add to the depth of combat and provide players interesting ideas, do i want 10 poor tank units or 5 good ones.

Another idea is to have training promotions, ie send a unit to do training, perhaps a cost of gold might be needed and they could gain experience for training.

Maybe as well as a worker unit you could train a slave unit (which could only be available if you have the slavery civic) that builds things faster and can be sold to and bought from other civilizations.

That's already covered somewhat by the choice of different units to build. Being able to buy promotions would make it too easy for a peaceful builder or financial civ to field a dominant army, without having to battle harden their troops. It would make it almost impossible to conquer those civs, as they would have few if any enemies and an army just as powerful as any warmonger's

@Gorman Truart
I think that's the point. A peaceful civ may have a strong army, isn't it?

Then what's the point of warmongering? As it is now you choose a path, and that path has strengths and weaknesses. Warmonger and you have a strong army and lots of territory but your tech and economy slows. Build peacefully and your tech and economy are strong, but your militaty isn't first-rate. Allowing peaceful builders to easily build a strong army will make that the best strategy and unbalance the game.

@gorman

well, make those promotions very expensive then. it's not that difficult. That means you have to be ridric (ridiculously rich) to be able to have really strong army.