No armour upgrades! Or at least not so effective ones. Maybe the same with Weapon upgrades; gives the player too much of an edge. Also, breaks the unit balance, esp. Archers lose their kill ratio.
Hmm, it does get me thinking... What if Armourer line buildings were just a requirement for units that are armoured... Would make things more sensible, really. As it is, Desert Archers or Genoese Sailors can have more armour than the Halberdier and still move fast compared to their slow...

Metalsmith line always seemed a bit awkward to me to begin with (firstly if you don't have metal then where is your units's stuff coming from, secondly if your metal is low-quality or something, then why can't you still just import better?). Maybe it should make production of units, particularly heavily armoured ones, cheaper instead, probably also should be way more evenly spread out across the map. Maybe there could be some otherwise ridiculously expensive units that would become affordable with Metalsmith.

Oh, reminds me of another thing. Loyalty of the province should depend on the quality of the garrison, not just the quantity. Doesn't seem quite right to me that putting a horde of completely incapable soldiers on patrol frightens the population. On the contrary, the presence of a huge useless army would further disgruntle people, if anything. This would further tone down emergence of huge empires as well.