With the amazing variety of armor styles available in the game, I would love if there was some sort of transmog system!
However, a dedicated appearance-collection system like in World of Warcraft could be more difficult to code perhaps.
A certain game that I thought did it extremely well, and simply was Terraria.
Characters have real equipment slots and "vanity slots".
Each vanity slot is bound to a real equipment slot.
Items worn in vanity slots do not carry over any stats to the character, instead they override the appearance of any item worn in the corresponding slots.
For purpose of fairness, perhaps these items slotted in vanity slots could not contribute to weight.
Terraria also had dye slots to further customize armor color, which would also be really nice but seeing as I don't know if the equipment models support this, it's not needed.
Oh and since I heard the argument that a transmog feature could help "hide" armor class in PVP, it could just be disabled when playing in the arena.
Of course it's not necessary and I am sure the devs have far more exciting things to work on.
But if this implementation is somehow dirt-easy to implement and enough people appreciate the feature, it would be pretty nice!