Balesir
Adventurer
I don't think you need to go that radical. Just give every armour an AC 2 lower than it is now and add proficiency as for every other application of it. Magic enhancements are additional, as they always have been in D&D.What if magical plate gave you DR 10 bypassable only with criticals? It would make sense if you're willing to alter what AC stands for.
The tricky one might be shields. But you could just have shield proficiency give a flat +1 (plus the bonus of the shield itself - 0 for normal and +1 for large shields), and have shields useful for other stuff - they were an offensive tool in historical WMA, after all.