I think its time for WoW to let the head, shoulder and waist armor be transmog available for everyone, no matter what kind of armor is (cloth, leather, mail or plate)
I know some people are against this kind of things because, for them, a priest using full plate like armor breaks their “fantasy identity”, and thats ok, but head, shoulders and waist should be perfectly fine. We already have plate armor for everyone (like the Orc, Mag’har Orc and Human hereditage armor, or Aberrus Priest set).; and the “a paladin with demonic armor doesnt make sense” doesnt work because paladins already have old gods, demon and undead armors, and warlock now have tree/plant armor set (Lighforged warlock draeneis, night elves warlock, etc).
Think like the Trading Post’s Class sets. They are only head, shoulder and waist, and it works. A Mage can use a helmet, or a heavy belt, thats not going to interrupt their cast; a Druid can use some shoulders from Hunter or Shaman armor because a lot of them are just leather, scales, rock or bones. Have you seen warlock and dk helmets? they are almost the same.
This will open a lot of new transmogs without breaking the “fantasy identity” and "the “casters cant use heavy armor because they must move their body and plate armor is too heavy”. Let the imagination fly 