I would love to see a tech system which acknowledge the environment in which a population lives. Environment and available ressources shaped regional technological progress in a way which is rarely portrayed in strategy games. For instance, something as important as chariot was invented by steppe herders, not by the early civilizations of the middle east which are supposedly more advanced. It only became very important in the middle east afterwards, both for agriculture and warfare.
During the late bronze age and early iron age, a big part of the economy of the steppes was actually based on metal mining and working (notably copper), with pretty advanced techniques (not just on pastoralism).
I've yet to learn how technological progress diverged* between steppe pastoralists and Middle East farmers during the Antiquity (as well as people from mountainous regions such as Iran, for whom both ways were important, or semi-nomadic desert people who should have pretty different evolution than steppe herders)
A tech stage system such as EU4 would feel bland, especially during early eras, as it would not capture how environment and ressources drive technology advances. I don't know if this reasonning applies for Antiquity the same way it applies for the bronze and iron age tho.
*may not be the right term, as technology spread between these regions.