I see an issue, which leads to a suggestion. I think Chainmail Profiiency and Plate Mail Proficiency traits make sense, but then I noticed something strange - trained units can wear whatever they want with no trait!

I've seen voiced by others an issue I agree with - trained units are just too good! Don't get me wrong, I like them - leveling them, designing them, their new abilities, everything. But the "hero xp issue" relates to this - trained units are way better than heroes below level 10 (and most heroes are unlikely to break level 10 in my experience, I have a sovereign to level up!).

So here's the suggestion - make trained units require these armor traits, too. If it truly requires training to wear plate mail, why shouldn't the trained units have to get the training to do so?

This would have the small effect of nerfing trained units at mid and late game - and I am just fine with that. This would also make leather armor more useful - less protection, but more room for specialized unit strategy. 

Bacco the Beggar is sick of watching me win the game, while he goes home with whatever maiden stayed until the tavern closed. He wants to be abroad, secretly experimenting with Quendar women and their scar stones!