The game does have "ruins" in many scenarios, which provide stone. The problem would be to take care of a more seamless transition between all kinds of buildings and their corresponding ruins. Maybe later on all these concepts could be unified in a coherent framework, but right now unbuilding/unproducing seems the simplest way.   The game already includes Loot from killed units. Realistically, to the victor should belong the spoils, but as you say, in a game like this it's hard to determine when a battle ends, thus individual opportunism is what's accounted for.

I would find it "logistically" more "realistic" and tactically more "interesting" if destroyed buildings, no matter if self-destructed the enemy's, would turn into "mines" containing the resources which have to be explicitly gathered and brought to one's storehouses. Also a battlefield could turn into "mines" with the "loot", like scouring the field after the battle looking for usable stuff, although it's hard to determine when a battle "ends". Would change balancing heavily against rushers/attackers in far away foreign territory though. Instead of "mines", the resources could also turn into those bonus-/treasure-item-like things, although those don't have to be brought to one's storehouses.  

You could check out what we did with  <SpecificName>Hastātus</SpecificName> for rome.  My understanding is early on they were similar to hoplites in kits and then they evolved into swordsmen.  So in p1 Hastatus = spearman and then it is replaced in p2 by Hastatus = swordsman infantry_swordsman_b.xml infantry_spearman_b.xml

Well the game already does it for "hoplites" they have a mixin that allows all hoplites to have phalanx formation.   <Identity>     <Classes datatype="tokens">Hoplite</Classes>       <Tooltip>Hoplites can use phalanx formation</Tooltip>       <History>Hoplites were the very symbol of Hellenic prestige and citizenship, armed with a spear and a large round bronze-coated shield known as an aspis. Armor was heavy, with bronze helmets and a cuirass of either bronze or linen, in addition to greaves. Hoplites fought in a tight formation called a phalanx, guarding each other with their shields while they attacked the enemy with their 2.5 meter spear or short iron sword.</History>      </Identity> just add a visable class     <Identity>     <VisibleClasses datatype="tokens">Athenian</VisibleClasses>     <Tooltip>A true citizen of Athens.</Tooltip>   </Identity>

I think the point could be to keep selectable options limited and uniform among civs, while allowing for actual differentiation and evolution if wanted. I've come across this issue so many times, and one has to end anachronistically simplify things, which could indeed be solved if "a unit can represent more than just the soldiers equipped in the exact same way". For example, the Athenians had hippotoxotai (cavalry archers) substituted by prodromoi ("scouts") in the first half of the 4th c. BC. With the proposal, both could appear, depending on progress, but the icon, or just the "generic" name, could be the same, and quite common for most civs (Mounted Skirmisher could be an option), but the actual model would change (among civs and with progress), have different stats, and the "particular" unit name also be written somewhere, but the "generic" name would tell you basically its function. Other examples I came across just this last week is that Alexander's mounted skirmishers went from javelineers to archers, and his prodromoi apparently from javelineers to lancers. And I guess it would simplify dealing with things like the Roman Army.   Regarding mixins, if you can irreversibly upgrade them such that they also change in the buildings that produce them, and show also their "particular" name somewhere, maybe it could work, but those working on it would have a better understanding on why it wouldn't.