A mechanism I'd like to see in Civ6 (if it ever happens!) is a true obsolescence of military units. Someone in another thread (and you must forgive me - I've forgotten where the post is) suggested that once a player/AI researches a tech which gives an upgrade to a particular unit or class of units then the player/AI should have their units upgraded automatically. I would change this to give the player the choice to upgrade or disband. And also, I would like to see some mechanism by which units that face an opposing unit which is two upgrades ahead automatically have their victory odds reduced to 0.01%, regardless of relative strength. To make this "realistic" it may be desirable only to impose this once the more advanced unit is from the Industrial Age onwards. Combined with stripping the ability of pre-Industrial units to collaterally damage Industrial/Modern units this would remove the frankly ridiculous (to my mind) situation of a bunch of trebs and knights counter-attacking and defeating a stack of Tanks/Infantry/Artillery. Comparing the weapons systems we can see that knights and trebs would be splattered all over the battlefield before getting remotely close enough to actually damage the more modern units. The 0.01% victory chance would reflect the relatively few occasions in history where units using massively outdated weaponry defeated modern units i.e. Redcoats defeated by Impis at Isandlhwana in 1879.