How many civs do you think can plausibly be added with this style. 10? 15?We have aoe4! This game has multiple civs that all play out with completely different economy structures, map control necessities and unit compositions with no mirror, thus adding to the feel of variety and IMO will make it harder for devs to nerf units across the board that will affect gameplay of all civs this way when a mistake is made and a unit underbalanced, the civ will suffer so bad they will see their mistake much clearer.
At some point your going to run out of mechanics to change.
and if you think to add less drastic changes then you end up back in age of empires 2