What about doing like Civ1 and Civ2, with a set of all unique units, but the same for each civilization? IMO the spirit of Civ2 particularly is in that fact of many heteroclit units put toghether within the same civilization, the civilization you are playing, it is to say any and every civilization in the game. Like that, each civilizatoin would have the war elephant, the hoplite, the legion, etc... i feel it amazing to play a mystic civ with so different and heteroclit units in the same pot. It draws the figure pretty well. Of course it does not fit what you reach mainly ShiroKobbure, but could be a kind of compromise. (like civ is familiar with)
But I would like, also, to be able to play more with the 'golden age', like being able to have more than one of them in an entire game. It may implicit several unique units, maybe one for each era (but it may be hard to find one for each and each civ, for example what ancient uu for america?) Several events could also trigger a favourable period, like the discovering of a new land and/or new ressources, a war with an important enemy, the end of a war, etc... I would add also that civ3 authentic 'Golden Age' is too weak in my opinion, and should allow the discovering of more techs to allow a given civ to really make the break. Of course there could be several kind of favourable peiod, already like in Conquest with the scientific golden age, it could also be an economic, production, and why not happiness only favourable period.